In particular, I can type "hel wor" to narrow to "hello-world" in a helm M-x session, but in selectrum, from what I can see with the defaults, it expects me to type "hw" - which is nice (and I believe helm also supports) - but sometimes I would like to type space separated words and have the candidate list narrow in sequence based on each being a partial match.
This was an especially noticeable deficiency when trying selectrum with projectile-find-file - I couldn't open my dotfiles and type "init" to find "init.el", as it seemed to want me to specify the entire path preceding it.