I'm actually launching a hosting review site right now (tonight) so it's interesting this came up. I've found this is very much the case across all hosting review sites.

I decided to shift focus towards working with hosts (any host) that want to be included in the directory, whether they had an affiliate program or not.

Of course if a host happens to have an affiliate program, I see no reason to not use it as long as your transparent with your users.

Plus it keeps the site free of regular banner advertising.