The number of aliens isn't the problem for me. I really don't like Dr Ree or the Efrosian and am not keen on several others, although I do like Pazlar and Torvig. You don't have to like all of the characters to enjoy a book.
My problem is part the 'variable' quality of the novels, and (probably moreso) that the exploration / alien world of the week stories dont seem that interesting anymore. Enterprise, TOS, TNG and Voyager did all that...a lot.
Superior exploration stories like Over A Torrent Sea and Voyagers Children Of The Storm are obviously good, but to be honest I'd still probably enjoy something else more.