I don't think Gary ever truly shed the OC job. He never really coached the whole team. He picked two terrible DC's to run the defense, and then, supposedly, hired Wade Phillips because McNair made him do it. I like Kubiak as an OC, but not as a head coach. I also think his offense became predictable and stale in Houston. Part of the problem was Matt Schaub, but part of the problem was the defense knew what plays Houston was going to run, which led to the pick 6's that Schaub threw.
Plus, it does not appear that Kubiak wants a HC job right now.
I think you're selling him short- Houston was an expansion franchise that won 2 games in the season prior to him taking over. He got them to .500 in his 2nd season and won their division twice in a row. Their record is 61-64 under him and 27-56 under everyone else. All this with Schaub, David Carr, TJ Yates (won a playoff game with him), Sage Rosenfels, and Case Keenum at QB. Sure, his last season was awful, but that was without Foster and Schaub for ~half of the season.
The Bills clearly need an offensive mind, and Kubiak is the best option IMO. If they can pair him with Schwartz, that would be a huge win.
Let me put it another way. I think Gary Kubiak, as a head coach, with a really solid DC can get you to the playoffs. I don't think he is likely to get you close to the Super Bowl. Were it not for the wonderful Marvin Lewis/Andy Dalton combination, Kubiak may not have any playoff wins in 7 years as a head coach.
Would he be a good fit for the Bills? Yes, probably because they have a good DC and defense already in place. But I tend to look at all coaching hires as "is this guy a championship HC"? With Kubiak, I think the answer is no. But that's just my opinion, having watched him coach 7 years in Houston. Buffalo could certainly make a far worse selection than Gary Kubiak.
The Bills haven't been to the playoffs in 15 years- obviously winning the Super Bowl is the ultimate goal, but you have to make the post season first.
I still think you're being too harsh on him. Let's be honest, your Texans weren't super talented, and it's hard to ignore how much better the team did after he took over. I don't think any coach in the NFL would have won a Super Bowl with that team, so it seems a bit short sighted to hold that against him. Overall I'd say the team exceeded any realistic expectations, which should be viewed as a positive for him.
He doesn't have any rings as a head coach (again, I doubt anyone would have with that team), but he does have one as the QB coach and two as an OC (and QB coach). He many not be a "championship HC" (yet), but I'm pretty sure none of those are available right now. He's a very good coach who would be a great match IMO.