As the infiltrator, i say this, along with vanguard, I cannot play. Adept, sentinel, engineer yes, and as many bad infiltrators as there might be, im darn glad to have a good friend who shines in the overwatch position. He calls out targets, shoots, warns, and provides cover fire shots to stagger and force ones like banshee's or atlas's to stagger. He likely ends up going down more than we do because of covering our hind ends that he misses one fellow that snuck up behind him while we we're sadly occupied.
Infiltrators tend towards certain roles, there are the cannons, they work for some, not for others. The assassins, the melee of the above, scary, and capable of their own nastiness. Another is the overwatch, which with a good eye, spends more time out of scope than in calling targets, and helping with things. In general, some races tend to fall more to others by synergy of race and class powers.
Our usual setup is AA (or SE) , KV, GI, and HS. We work very well because of the nature of our fellow on overwatch. For others, if your more used to bunkering, then you may want to look at the infiltrator as your long range fellow over damage, with the focused synergy, in general, yes, the bad class examples exist (to my shame to have been instant killed as vanguard with a brute). But really, its the person, though weapon can shape much more so how a class can effectively work.