Definitely have some concerns with sidechains. They may be a valid use case but they are not Bitcoin (though they are being sold as if they are) and are therefore a compromise. My biggest concern though is that they are being sold as a solution to a problem we are having but as yet, they are untested vaporware.
Bitcoin was hacked very early on, and any new additions to its ecosystem need extreme testing before they are incorporated in case they introduce weaknesses. Vaporware cannot be tested until it's completed, and its inventors shouldn't be arguing for its inclusion until they have the let others do extreme testing on the completed code to find any weaknesses they may have missed. Exchanges get hacked and the hackers responsible will go after sidechains.