On the contrary, the Longinus has extra firepower. Yes, you're limited to moving one MP unless you fire or eject it, but you do have an option for extra firepower with the suit. If you cannot make use of it, you still got a suit that is equivalent to other general purpose combat suits (no LRR but David Gauss has a bit more range so it sorta balances out), if you can make use of the SRM, you have a superior suit.
Other GP suits do not have this option, and thus aren't quite as good.
Fa Shih is a good suit but at 7 armor it is vulnerable to the quite common large laser which is a mark against it as a general purpose combat suit. Not a major issue (could be worse, like the Fenris), but it is there. (Also under standard rules mines are out of question so this reduces the value of Fa Shih, but admittedly this isn't really a fault of the suit.)
EDIT Personally i don't regard "GP" here as meaning solo combatant but just denoting a suit that isn't really specialized. Naturally BA work in conjunction with other units.