The latest Magento 1 security patch SUPEE-8788 contains 17 APPSEC updates, so it is very important to apply it as soon as possible. On the other hand, these are many potential BC breaks, and given the history of patches over the last year I would not apply it carelessly.

Good thing is that this time there are no frontend templates involved, so it looks like we don't need to patch all our themes. This is only true for Magento 1.8 or higher.

Nonetheless: Did you encounter any compatibility problems or bugs after applying the patch?