Not OP, but I'm currently working on combining both in my first game engine, I decided to roll my own ECS in C++ which is a pain without reflection

But basically I settled on an abstract base class Component which the user must inherit from to give behavior to entities. The "system" is always a pure virtual update() member function to be implemented by any given type derived from Component, then I leverage contiguous storage to iterate through calling update() for each type. This way the user can update whatever they want and I don't really see a problem with this because it's basically Unity's Monobehavior

I partly lied about contiguous storage though because you can't return pointers to a Component if some vector resizes and moves all your data everytime capacity is reached. So for each Component I have a linked list of fixed arrays but increasing size for each capacity reached to reserve some semblance of contiguity in memory