Yep, that plus the fact that the other team sees who is coming on to the field, as well. It won't take them long to figure out who is on the field and what "package" is on the field and adjust accordingly.
There are no gimmicks that are consistently successful in the SEC. The players are too good and the coaches are too smart to fall for that more than once.
Adjusting to the package is one thing, adjusting to the play called by the D is another. You could have the same personnel execute a blitz or drop back in coverage. You might even try to associate a package with a particular play early in the game only to change it late.
It seems to me that coaching will determine how successful or not the package approach is. I would not call it a "gimmick". The NFL does it that way all the time.
An example of how NOT to do it....put Leake in at QB and the whole WORLD, let alone stadium, knows he is going to run it.