Push fit or compression caps? Are they reliable?

Solder fittings are the gold standard in copper, however compression fittings are generally the next level down and very reliable. I would stick with those over push fit as these can pop off.

John Guest speedfit (my preference) push fit caps are a good solution for temporary caps but as the other poster has said, best to use end feed or yorkshire solder cap or compression cap for more permanent applications. I have never known a push fit fitting to come off, I always use JG speedfit fittings which are great and have the added benefit of being easily removable if required. I also like the fact that you can use a collet clip on them once fitted to ensure that they can not be accidentally knocked off. Hope this is of some help !