The main appeal for me is the well-designed avoidance of the valueless- by-exception state.
2009: Boost library components often form the basis of new standard library components
2019: Standard library components often form the basis for new, improved Boost components
If I remember rightly, former senior Boost folk tried to stop std::variant shipping with its valueless-by-exception design, demonstrating that it could be easily avoided using a fallback to a double buffer design. WG21 were so keen to ship variant in C++ 17 that those concerns, amongst others since defect remedied, were brushed off, and we are where we are at. It could be worse. I think it's entirely right and proper that Boost ought to ship fixed standard library types where possible. In many ways, Boost always has - std::deque is not useful in portable code, whereas Boost's containers have the same behaviours everywhere. I'd just love if WG21 would deprecate the current containers, and standardise new non-allocator intrusive ones instead, perhaps with an inefficient convenience wrapper. Niall