While mod_alias is faster than mod_rewrite (because it’s part of Apache’s core), it lacks the power of mod_rewrite which is designed to use Apache’s regex engine. Yes, RedirectMatch uses regex but it can’t use conditionals.
I’d either:
Rename that directory with the reviews link
OR
Learn mod_rewrite to control the redirection (and provide an exclusion to the {whatever}/reviews URI).
IMHO, your redirect is not sufficiently different to not loop. Okay, at least that’s what I suspect is your problem.
Using mod_rewrite, you could add the /1 using:
RewriteEngine on
RewriteRule category/product-name/reviews$ category/product-name/reviews/1 [R=301,L]
The key to this NOT looping is the “end anchor,” the $ after reviews in the regex. Since your redirection cannot be served by Apache, place this before your CMS’s mod_rewrite block.