There is a pretty good variety of enemies in Devil May Cry 4, ranging from the weak Scarecrows to the insanely powerful Blitz demons. However, I don't think that the list of enemies, nor how the enemies in the game are used are perfect. It is a little hard to pin down exactly, so I will tackle the various enemies in Devil May Cry 4 one at a time.
These are the successors to the Puppets of Devil May Cry 1 and the Hell reapers of Devil May Cry 3; they are weak enemies designed to be fought in large numbers throughout the game. Unfortunately, the Scarecrows just don't seem to do the job as well as their precursors. First of all, the Scarecrows only come in two main varieties, an Arm type and a Leg type, which are difficult to distinguish from each other since they both fight in more or less the same manner. In comparison, the Puppets of DMC1 came in blade-wielding, dart-throwing, and gun-toting varieties, as well as in the slightly stronger Bloody Mary variety. In DMC3, there were seven kinds of Hell reaper, each of which was both visually unique and had distinct powers. This lack of variety really hurts the Scarecrows, since most Scarecrow battles end up feeling the same.
The other problem with the Scarecrows in DMC4 is that they don't appear very frequently in the game. It is actually a lot of fun to tear through large hordes of weak enemies using your large array of flashy attacks in a Devil May Cry game. That is more or less the reason for the existence of the Scarecrows and their ilk. Yet, Scarecrows don't actually appear all that frequently in Devil May Cry 4. While they appear in just about every major section of the game, with the exception of the first few missions they only appear in a room or two per mission, if that. In comparison, just about every corridor and transition room in DMC1 or DMC3 is stuffed with fights against similar enemies. Part of this is due to DMC4's tendency to build stages around 3 or 4 major fights against stronger enemies with little in the way of optional fights in transitional areas.
Assaults and Frosts:
With DMC4's emphasis of having a few major battles per mission, the lizard-like Assaults end up being one of the most prominent enemies in the game, particularly in Son of Sparda mode, where they al but replace the Scarecrow. They are designed to fulfill the role of an enemy that is a bigger challenge than the Scarecrow, but still weak enough to be fought in groups of three or four. In that sense, they are somewhat comparable to the stronger varieties of Hell reaper from DMC3. However, while the stronger Hell reapers came in a wide variety of specialist forms, the Assaults come in only one form that tries to do everything at once. They throw long-ranged attacks, perform a variety of mid to long range dashing attacks, have decent melee capability, can perform a screaming attack at short range to stun Dante/Nero, and can block both gunfire and melee attacks. This all-in-one nature of the Assaults doesn't really help them as monsters. Since they can fight using a variety of moves, there isn't really any kind of strategy for fighting them, so they end up being more annoying than anything. At the same time, they always fight the same way and rarely come alongside anything else, so fighting them gets old after a while.
The Frosts are really just elemental versions of the Assaults with a greater emphasis on power and defense over speed, so a lot of my comments above apply to them as well. Oddly enough, they are introduced much earlier in the game than the Assaults.
Chimera Seeds and Chimeras:
I actually like these things. The Chimera Seeds are almost unique among DMC4 monsters in the thematic nature. The Chimera Seeds only appear in the forest area, and are directly linked to the boss of the forest, which successfully ties together the enemies in the area, the area itself, and the boss of the area together in a way no other area of the game is. The Chimera Seeds themselves do something unique: they attach themselves to other enemies and make those enemies stronger. However, I much prefer it when the Chimera Seeds start out unattached, so I have the chance to try and prevent some of them from attaching to other the enemies. For too often, enemies come with Chimera Seeds pre-attached.
I really have no idea why the designers of DMC4 used Basilisks the way they did. Basilisks fulfill a pretty basic and necessary niche of monster: they are relatively fragile enemies that specialize in ranged attacks. As such, they should have been a major enemy that appears alongside Scarecrows and Assaults for most of the game. Yet, the Basilisks are introduced only near the very end of Devil Hunter Mode, appear in only a few places even in Son of Sparda mode, and generally only appear in packs consisting of only Basilisks. I really have no explanation for it.
Once again, this is a monster that only appears in very few encounters. Yet, they are a pretty interesting enemy, since they can picked up by Nero and thrown at other monsters for a pretty powerful attack. At the same time, their presence can make a tough fight much harder, since they are distracting and can do a fair bit of damage. They really deserved a bigger presence in the game.
As far as I can remember, these guys only show up in a pack of three in one encounter in the entire game. Their presence is so limited that I don't even know what to say about them. Like the Gladius and Basilisk, these guys seem to have been designed primarily for their role as weapons used by Agnus Angelo. Outside of that role, they don't seem to have much point in DMC4.
Bianco Angelo and Alto Angelo:
I can't deny that the Angelo-type enemies are some of the coolest enemies in DMC4. The Bianco Angelos work well as a mid-tier enemy designed to be tough until you get used to their quirks. The Alto Angelos work well as elite opponents on their own, and can transform a group of Bianco Angelos into what is practically a mid-boss fight. Yet, I can't shake the idea that there should have been a third kind of Angelo, such as an Angelo similar in strength to the Bianco that was designed for ranged combat. That would have given some more variety to the early/mid game Angelo fights, and would have opened up a greater variety of Alto Angelo squads. Alternatively, it would have been nice if the Angelos appeared alongside the weapon type enemies (Gladius, Cutlass, and Basilisk) more regularly, and even better if they could have synergized with them more. For example, maybe an Alto Angelo could pick up a Cutlass and use it as a weapon, similar to how Agnus does.
Mephistos and Fausts:
Honestly, at this point I just plain hate these things. There is only one strategy that works on them: shoot them until their cloak falls off (which can take a frustrating amount of time), then slash them while they are vulnerable until they die. At the same time, they are too powerful to casually group with other enemies. The one time I fought Mephistos and Frosts at the same time, I discovered that it was almost as tough as a boss fight. As such, you only fight Mephistos in groups of two or three Mephistos, with a Faust occasionally thrown in for a particularly tough fight. This makes fights against Mephistos tedious, annoying, and boring. I much preferred enemies like the Bloodgoyles of DMC3, which were similar, but weak enough to be paired up with other enemies.
This enemy has so far only appeared by itself as a really difficult solo fight. I have died or nearly died every time I have fought it. While there is only one real effective strategy, the enemy is rare enough and strong enough that I can't really complain. The third time I fought a Blitz, I got to use a Gyro Blade to fight it, so I can't fault the designers for not trying to spice up battles against the thing.
After writing this much, I think the recurring issues of enemy and encounter design have come out into the open. In short, while the game has a wide variety of interesting enemies, the enemies don't complement each other very well. You fight most enemies in groups consisting of just that enemy type, with occasional exception. Most of the enemy-types in the game are fairly resilient enemies with a wide range of both long and short range moves, which means that they don't have glaring weaknesses that the player can exploit or that other enemies could cover for in a mixed group. A such, there is little strategy to be found in determining what order to kill the enemies in. The lack of variety also means that fighting these enemies gets a lot less interesting after a while.