Sure.
Rails: Rails is always moving fast. To keep up to date you will have to go through some migration pain from time to time.
Sinatra: Nice little framework but you very probably have some redundancy with your Rails code. Also you need to decide when to use what.
MySQL is nice and battle proven but as there is also Redis added to the mix. So one needs to decide when to use what. It's not that there is one DB that you work with, data is split into two different systems. Solr needs to play nice with both dbs I guess? Sidekick looks like a natural addition and is pretty much a no brainer I guess. There might be things to consider when updating Rails that affect Sidekick.
One might assume that the frontend part seems to be decoupled from the backend part but as there is Rails in the backend I can imagine that some parts are rendered server side and some parts are backbone + mustache (mustache is another thing added to the mix as backbone has its own templating language that seems to be replaced here). So this is why I can imagine that it is not an easy task to maintain the codebase as there are more technology blocks involved than for example plain rails + mysql.
I loved 8tracks. Got several friends to use it. But about 6 months back the Android app update screwed up the whole experience. Skipping songs, loading problems, sign in not working - I waited for several weeks but the experience was still awful and I had to stop using the app. I think I will try using it again though, because the content is just amazing.
