Gotta agree with DS. If a game gives you the option of playing with several different types of weapons, it should make each weapon type be at least viable for the vast majority of the game. That is, you should be able to at least FIND a type of each weapon in the early game and each type should be at least powerful enough to scrape by the enemies.
The types don't have to be balanced or anything, but they should be viable. I actually think Arcanum is a pretty decent example of this, as crappy as the combat is - it is totally possible to win the game as a warrior, a gunfighter and a mage. Sure, a mage with Harm will win the game ten times easier than the other two types, but each type can fight and win from the beginning of the game to the last boss.
All FO games but NV and Tactics have so far failed on this account, IMO, as small guns really are the best, being far easier to find and feed with ammo than EWs and lighter and less ammo-consuming than BGs, who also tend to lack the variety in weapon types and damage penetration to make them viable against a lot of enemies such as the Enclave in FO2 and 3.
FoT was a special case in that EWs were pretty weak for most of the game but became KING once you began running into robot enemies, and big guns definitely had their place too.