Maybe just a matter of developer discipline, but in my experience people tend to create large stacks of 3+ PRs that then take a while to resolve. Yeah sure, without these tools the code would also exist somewhere, but at least you don't have your pull request list full with PRs that aren't yet ready because the upstream PRs haven't been approved yet. You anyways have to (or should) review things stricly serially, so the additional PRs existing are not super useful either. Maybe as context for the reviewer to see the future work.
Also for some reason these tools seem to encourage people to put unrelated changes in the stack that could be based directly on master. Probably because they are anyways working on their stack and integrating some unrelated fix they just did into the current stack is easier than rebasing/changing your current context. But that's just a minor pet peeve.
That said, I would still like a tool that lets me manage stacked branches just for myself. Not exposing them as Github PRs or so, but to organize my work into different branches. Executing a chain of rebases of branch N -> (N-1) can get quite annoying manually. Probably some arcane git magic to (interactively) rebase branch N->(N-1)->...1 in one command exists already.