This can be a problem because the mechs are often the most predominant and evocative face of a character in a mecha game or series. In Front Mission 1 and Front Mission 4, the player only sees facial portraits of the main characters outside of their mechs. I don't think the player ever even sees the face of of the main characters of Armored Core games. However, even in mecha anime series where the main characters get more screen time than their mechs, the machines they pilot are a big part of their image. Furthermore, mechs often serve as the faces of countries and organizations. In the original Gundam series for example, all of the Mobile Suits piloted by Zeon pilots had distinctive mono-eyes that moved around on tracks. They all had a unified look. It is easy to tell apart different sides on a battlefield just by looking at their mechs.
However, the Front Mission series lacks this. When I was playing Front Mission 4, I had real trouble identifying what mechs were appearing in the cinematic scenes. Since part of the plot involved one country using certain models of wanzer to pin the blame of an attack onto another country, this was actually a pertinent story detail. Even though the Front Mission world is divided into 3 major factions, none of these factions have clearly differentiated mechs. For example, in one early cutscene in Front Mission 4, I noticed that my allies' mechs had integrated gun-arms and no discernible head. However, I later fought a completely different, completely unrelated faction later in the game that used mechs that looked nearly identical.
A big part of the problem in the Front Mission series is that the player and the enemies are constantly upgrading to completely new mechs. Since the player has to change mechs every three battles or less to completely new models, you don't have much time to get used to a certain appearance for your characters. As it is, I can barely remember what my character's mechs even looked like past their paint schemes and general size. And since the enemy mechs looked different every time, I never was able to develop a good idea for what they used. All of the mechs in both Front Mission 1 and 4 kind of blur together. There is one major exception to this in Front Mission 1: Driscoll, who pilots a distinctive black wanzer called the Type-11DS which is larger than a normal wanzer.
However, there is one element of the mechs in Front Mission that I can clearly remember: their weapon load-outs. Since what weapons a mech is equipped with is highly dependent on the character's stats, every character tend to use the exact same kinds of weapon on a constant basis. So Elsa used both a machine gun in one hand and shotgun in the other, while Darril used a shotgun paired with a melee weapon. This kind of consistancy helped a bit, and helped identify various characters.
Another set of mecha games that I have to draw some comparisons to are the Xenosaga games. In Xenosaga Episode 1, the characters have their own mechs called AWGS that could be customized to use different hand and shoulder weapons, much like a wanzer. However, since the mechs themselves did not change in appearance but are instead simply upgraded, they are much more memorable. However, since the stats of the mechs were not dependant on the stats of the pilot, the mechs could be swapped around between different pilots, and only one character ever piloted a mech in a cut-scene, they tended to feel tacked on instead of serving as a second face for the characters. So while it is improved over the Front Mission series, it still can do better.
From what I have read on GameFAQs, it seems that Front Mission 5 did make some improvements on the system. In Front Mission 5, there are much fewer mechs, but they can all be upgraded and refurbished to have better stats. So, a character can go through much more of the game using the same mech if the player so chooses. This approach does maintain the customizability factor from older Front Mission games, but does allow each character to be better associated with certain mechs.
The next thing the Front Mission series needs to do is make it much clearer what factions use what machines and make every faction stick to those machines in cut scenes. That consistancy would help give the UCS, OCU, and EC more distinctive faces, so it is obvious what faction is appearing in every cut-scene. Giving major plot characters consistent and distinctive mechs (like Driscoll's Type-11DS) would also help.