There are some legitimate reasons to dislike loop. It's a high level construct, yet it has unspecified behaviors. A program can be nonportable on account of some manner of using loop. It's been the case in the past that Lisp applications ended up carrying their own private loop implementation which would behave the same way everywhere.
Today there is a related discussion in comp.lang.lisp: https://groups.google.com/d/msg/comp.lang.lisp/U5LqiM5nKq8/M...