Oh man, the weapon restrictions in ME2 is by far my least favourite part of the game. Limiting all "caster" classes to Pistol/SMG for two thirds of the game is just so dull. It wouldnt have been so bad, if the guns were any good - but they aren't.The main downside is that like most hybrid classes your weapon selection is shit: limited to SMGs and Pistols in a game with a really anaemic selection of both and no baseline ammo powers to balance that. Once you hit the Collector base you can get training in one of the other three weapons - I personally chose Assault Rifles - and once a loyalty mission or two is done you can get an ammo power to round out your damage.
The Predator Pistol is weak, and the Carnifex has so few bullets. The Shuriken SMG fires in bursts, but still sprays everywhere, and the Tempest is inaccurate at basically any range other than in your face. If you have the DLCs, you can get the Phalanx pistol, which is a stronger Carnifex with slightly more ammo, and the Locust SMG with is actually really accurate - but whilst this improves the situation slightly, needing to spending real money for competant weapons isnt exactly much of a solution, and it doesnt stop the game from feeling pretty monotonous. Its why I have always really disliked playing the Adept, Engineer and Sentinel classes in ME2.
If the same system was ported into ME3, it wouldnt have been too bad, because at least that game has some variety - but im so glad it switched to a weight system, and just let you pick what weapons you want to use.