> some kind of bucket-optimisation (multiple elements per allocation, instead of one element per allocation) should also meet all constraints the standard sets for this type

Pretty sure that makes it impossible to implement merge without pointer/iterator invalidation, which is a constraint imposed by the standard (https://en.cppreference.com/w/cpp/container/unordered_map/me...).

> it would be good if the person I originally responded to came out and defended his POV

See https://probablydance.com/2017/02/26/i-wrote-the-fastest-has... if you want numbers, or for example this talk by the same author https://www.youtube.com/watch?v=M2fKMP47slQ.