Since Version 1.22.0 Embers Unofficial Extended Life uses Mixins to fix issues with other mods. If you are missing MixinBooter, you are likely to encounter imcompatibilities with other mods. To fix this, make sure you have MixinBooter installed. If you are playing a modpack with Embers UEL, make sure the modpack includes MixinBooter. I know that there are a alternatives to MixinBooter, but I don't recommend using them as they are neither used nor tested by me.
Sledgehammer is a mod that fixes various bugs in a lot of mods. The original Embers is one of them as it hasn't been updated in a long time. Embers Unofficial Extended Life has fixed all Ember-related bugs. If you are using Sledgehammer, make sure to disable the Embers fixes in the Sledgehammer config.
If you are experiencing issues with another mod, please check the compatibility section of the README.md. If the mod is not listed there, feel free to open an issue and I will look into it.