I think my main complaint about Rails is that it's pretty heavy weight. Lately we did a Sinatra app because we didn't really need anything that rails was giving us. Fairly quickly I realised that I didn't need anything that Sinatra was giving us either. Rack was fine.
I think especially after your app gets to be a certain size, you are either writing your own framework, or fighting with the framework you chose at the beginning. It's just the nature of the beast. From there it's more of a training issue than anything else. You can hire "Rails developers" (or whatever developers), but you can't hire people who are specifically trained in a system you've built. Depending on the kinds of developers you hire, this may or may not be a problem.
Personally, I like writing code and I like small, light systems that are easy to move around in. Usually that means trying to avoid large frameworks. However, not everybody I've worked with is comfortable with that experience, so normally we pick things people have worked with before. Either way it's not a big deal in the end.