At Ubisoft/RedLynx (2015) we developed a new ECS system for our new engine. It had super nice query API. The implementation was a massively complex variadic template monster. Only a few people in the studio could maintain it. Debugging it was hard. Compile time slow. Thread...

At HypeHype we are pushing unfinished features directly to dev branch behind feature flags. This improves iteration time compared to long lasting branches, and avoids regressions when people land their long lasting branches at the same time (never cross testing them).

Unknown object life time or owner is a code smell (ref count, shared_ptr). In most cases you should be able to define a clear owner for each object. In multithreaded environment custom destructors are very dangerous with ref count schemes. Hard to know when destructor is called.