Standard Stallbreaker is utterly useless against any well built stall team; it falls completely flat against said stall teams AND it doesn't do much against offense aside from fish for favorable switch-ins by spamming WoW/Taunt. At least Perish Trap Gengar kills stuff on stall. I still maintain that if you want to beat stall, you're better off using a wall breaker to punch holes--or just make better plays. Stallbreakers are so passive though; they're really only good at slowing things down. They're not going to make a stall team crumble like a wall breaker.
So--to answer your question--it's less situational because it at least accomplishes its task at breaking stall teams down. Neither of them are going to be worth it if you're concerned with beating offense--either way you're dedicating a slot to help combat stall. The difference between them is one will actually kill things and the other will just stop them from doing their job for a turn or two. I think you may be underestimating the gravity of removing a Chansey from a stall team--if you do that, your special attackers can pretty much run all over the stall team.
So--to answer your question--it's less situational because it at least accomplishes its task at breaking stall teams down. Neither of them are going to be worth it if you're concerned with beating offense--either way you're dedicating a slot to help combat stall. The difference between them is one will actually kill things and the other will just stop them from doing their job for a turn or two. I think you may be underestimating the gravity of removing a Chansey from a stall team--if you do that, your special attackers can pretty much run all over the stall team.