The 3-3-5 on its own is not what I would call a gimmick defense. One also should consider that most teams are rarely in their base defense, so it's somewhat of a misnomer. If you're running a 3 man front with a LB that walks up to the LOS and plays there 90% of the time, you're basically running a 4 man front. Too many people make too much of the name of a defense and go off of that.
Now, what I will say is that the way UM runs their 3-3-5 defense is kind of gimmicky. It relies on chaos and smoke and mirrors. Gap integrity is not important. There seems to be little rules. Since Hauck has been back and ran that defense, it doesn't seem like there's a lot of variation to it, but maybe that's just because I don't see it as much as others here. To me it very much seems like the defense is the defense, and they're not going to change how they run it with some few exception. I think it works when you play teams that you're more talented than, and when you play teams that you are not familiar with. It's one of the reasons that I think MSU matches up well against UM under this tenure...they can match the talent that UM has, and they see it every single year. It's an advantage MSU has, imo.
To be blunt, I think the defense UM runs, and UM just being UM, is one of the reasons why they'll likely always land in the quarters or semi's, but if they play against a team that has stronger line play or can match them in talent, they'll lose. That's just one outsiders take.