Firstly, that strategy works well only against cerberus. All other factions actively try to flank you.
Sometimes, even Cerberus flank you or they troll spawn behind you. But it's rare - may be 1 in 10.
The reason it works well against cerberus w11 is because of the nature of enemies. Phantoms and goons tend to take the shortest route and rush you. So, they keep respawning at the top of the ramp and quickly rush you.
The reason it doesn't work well on other waves is because of the lack of dragoons (and lack of phantom spam to an extent) - as simple as that. Troopers, centurions, nemeses, engineers fight from cover and staying at the bottom of the ramp is not useful as you won't have direct LoS most of the time.