Do some programs not accept process substitution for input files?
Process substitution creates a named pipe. You can't seek into a named pipe.
Yes. I've noticed the same thing in other programs. For instance, it doesn't work in emacs either. It gives "File exists but can not be read". And it's definitely a special file, for me /proc/self/fd/some_number. And it doesn't work reliably in either less nor most, with default settings.
For most:
most <(/bin/echo 'abcdef')and shorter displays nothing. Longer values truncate the beginning. less apparently works, but only if you specify -f.
I find zsh's = much more useful in practice. It's syntactically the same, except = instead of <. But it just creates a temporary file, so support doesn't depend on the program.
EDIT:
I found zsh uses TMPPREFIX to choose the temporary filename. So even if you don't want your real /tmp to be tmpfs, you can mount one for zsh.