Let's reason through this together.
First off, answer your own question.
It all comes down to talent, right?
How are the Bills talent wise?
Their special teams really doesn't need any justification and Spiller will merely add to that.
Defensively however they made the switch from the 4-3 to the 3-4. Running a 3-4, for obvious reasons, puts the strength of your D in the hands of the LBs. Does the team have the LBs and the depth at LB to have that be the strength of their D?
How about on the line? Do they have the types of DEs and NT that are required to run a 3-4 effectively?
IOW, is Stroud your typical NT in a 3-4? Schobel won't be here and he wasn't a good fit at 3-4 DE anyway, but likewise, is Kelsay?
If not, who on the team will play those positions and do it such that it clears the way for the LBs?
OLBs are key in the 3-4. Who will be our OLBs and do you have faith that they will excel in that role in the 3-4?
Secondary is secondary either way.
On offense, why will we be better? Just because Spiller's around? While good, is he personally really going to make all that difference?
What about the WRs? Who starts besides Evans?
How about TE?
Line, meh, we've made some acquisitions, but just as in past years, the names aren't as big, so why should we hope for instant improvement immediately, or "out of the box?"
And say what you want about coaching, but both coordinators have never been coordinators before. That's not good no matter what systems you are running.