Minor thing, but could you please use std::all_of?

Make it const please.

No else after a return statement.

God I wish we had good support for clang-format. Could you please remove whitespace between [=] and (bool defaulted)? Sorry for annoying you with whitespace issues. :/

Do we actually need this connect? I assume that signal dataChanged is emitted when the model is loaded.

dataChanged signal is only emitted in EffectsModel::setData

This inline comment hasn't been addressed.

I asked you to remove a else after a return statement, i.e.

if (effect.enabledByDefaultFunction && effect.status != Status::EnabledUndeterminded) {
    return false;
}
if ((bool)effect.status != effect.enabledByDefault) {
    return false;
}
return true;

Oh, you're right! You could also consider using one of QAbstractItemModel's signals instead, e.g. modelReset. Perhaps EffectsModel::loaded() should not exist in the first place.

I misunderstood what you meant, in my last change I added :

} else {
       return true;
 }

Should be good now

I suspect this lambda would have benefited from being a single return statement (not 100% sure though)

Urgh! C-style cast, please don't. Should be a static_cast or something.

Agreed, using it in modern C++ code is not good but on the other hand we have (bool)effect.status != effect.enabledByDefault just right above this method. If my memory serves me well, the c-style cast is leftover from the monolithic compositing kcm. This minor issue can be addressed in another patch though...

Sure, can only be in another patch anyway, that one got merged anyway. ;-)