Even when using a pool allocator, you still have unnecessary indirections which is expensive. One of the benefits of C++ is the ability of being able to allocate subobjects inline with the containing object or array. By forcing indirection, allocating subobjects requires navigating a potentially deeply nested tree.

This is true, and like I said above, it's not a method I prefer to use, it's just something that is commonly seen in projects that disable exceptions in order to avoid 2 phase initialization.

There are definitely things to be aware of before adopting such a pattern, or when trying to optimize code that uses it.