LogFAQs > #973019330

In many Gundam canons the limitation is that they require some level of New Type-ness from the pilot to be exceptional. The RX-78 line also often runs out of ammo and/or energy in the series; particularly for the beam rifle.

In 0079 the Gundam project doesn't seem to be particularly expensive or anything. It seems a lot more like a last ditch effort to get ahead of Zeon since the Earth Federation doesn't have time to catch up to Zeon's number of mobile suits in a conventional way. The EF is stuck with a bunch of mobile ball's and guncannons and shit until late in the series when the GMs start to hold ground against the worn down number of Zakus.

As for actual scientific weaknesses of mobile suit/Gundam design, yeah there's plenty of places to go after a Google search. It's kinda lame that they make no sense as real world weapons.