What specifically makes a fish difficult for beginners?

I haven't ridden a fish yet and I've heard many, many times from posts on the board here that "fish aren't easy for beginners" but I never have found what specifics or aspects of a fish board make it difficult for beginners. I know they are short (standard retro fish is around 6 ft or so), but they catch waves very easy, usually closer to 3 inches thick so they are easy to paddle out/bouyant and usually pretty wide which usually I read is a characteristic for being a very stable board versus a wobby 18 inch wide shortboard/thruster. Can some one please tell me why beginners would have significantly more trouble on a fish vs an 8ft minimal (your standard beginner board)? Is it just the lack of length making for a smaller target zone to put your feet onto during your poppup to get balance? That is the only thing I can envision so please give me a better understanding so I stop wondering (and newbies stop asking about should I get a fish). Just saying "they aren't good for begginers" doesn't do much for me. I want to know why.