My primary gripe with mecha lies not in their design in d20 modern, which I do consider flawed. It lies in the fact that they even exist in-game in the first place as anything more than glorified power armor. Mecha are best suited for an infantry support role. They're too top-heavy to carry the heavy main guns that a tank of their era could handle, and if you can build a mecha that can run and jump and FLY, whats to stop you from building a tank that can go 150 kph, turn on a dime, and HOVER? Seriously, D20 future gives up how many pages of Mecha, and how many pages of vehicles? The one reason ingame to include them is the fact that despite their high target profile and lower firepower capacity the can carry a missile or demolitions charge, and they might be fast and flexible enough for infiltration missions. Wow. Really, this is just in there for the OMGWTFBBQ cool factor.
I don't ever plan on using a mecha larger than Large. Ever. But thats just my hatred for things like Gundam and Battletech showing through because of how the tanks in those settings seem to be designed for mech target practice.