Using RF is usually a good way to get your soldiers killed on Very Hard. This is because your weapons do about 25% damage (while the aliens do quadruple damage), and the aliens can soak up mounds of firepower consequently (especially the armoured ones), allowing them to retaliate for a kill. If you must do this, the door in question should be camped with at least two people set to discharge the most damaging firemodes (particle cannons on charged mode, plasma blasters/rifles on spray, etc...). Otherwise (that is on lesser difficulty settings), door camping is more often than not useful as you can take most aliens down in an inital salvo, and survive any fire they manage to discharge.
In multiplayer door camping can be effective; as long as you're far enough away from the enterance to avoid getting flashbanged/naded. Any smart player will typically lob a flashbang or grenade salvo into a suspect room before walking in. You should also have at least two men camping a door. This is because:
A: Your chances of firing before the enemy can engage in hostile activity or withdraw are doubled.
and
B: In the event that one of your soldiers dies or is FBed, the other soldier can retaliate and/or cover.
Of course, they should be spread as far apart as possible to minimize the impact of flashbangs and splash weaponry.