It's typically better against the spread teams in college as it gets another "space" athlete on the field to cover sideline to sideline better. There are also a ridiculous amount of blitzes and coverages that a team can run from 3-3-5 that can be easily disguised, and it can really be tough on the opposing OL to know where everything is coming from. Like any defense, though, it has its weaknesses that good OCs exploit.
You're right that it's used by teams lacking the personnel for an extra DL, but it isn't just about that. It's a different scheme with completely different responsibilities for the "front 6". Each player in the front 6 is assigned one gap on every play, regardless of whether they are rushing or dropping into coverage. That accounts for all 6 "natural gaps" on a given play and then you have the two hybrid OLB/S positions that typically have contain or "D gap" responsibilities, in conjunction with their flat, curl or man to man coverage responsibilities.
Check Charlie Strong's record as a DC and his time at Louisville.
From Grantland: "As with most schemes, age has exposed many of its weaknesses, and many of its leading practitioners, like Charlie Strong, have moved on to other fronts and use it as only a subpackage"
I think I prefaced that in my original post. No exotic offensive or defensive scheme has success forever, but the 3-3-5 was proven to be successful for a period of time when it was first implemented. It's definitely not an every down defense anymore, but then again, there aren't many successful teams in NCAA or NFL that only use one defensive front, anyway.
