Nothing ... lots of people use them and love them. I started out with a pair of Apollo Biofins, and loved them too ... until I reached a point in my diving where they were not the appropriate tool for the job I wanted to do.
What one needs to realize is that like most things they have advantages and drawbacks. When people point out the drawbacks, it's inevitably taken by some as a knock on the fin. It's usually not ... but rather just someone pointing out that they're not the panacea they're often marketed as being. Sometimes the person pointing that out is someone like myself, who enjoyed them in the limited capacity they're useful in, and recognizing in the person I'm replying to someone who wants to take their diving beyond those limitations. But rather than taking it in that context, some people ... usually people who own split fins ... want to take it personally. Can't help it ... like a lot of equipment, split fins work well for the recreational diver who doesn't want to work that hard, who isn't going to be pushing heavy loads, who wants a fin that's easy to kick (perhaps someone who cramps easily or isn't in great shape) ... and for that person, split fins are the appropriate choice. If you want to learn some precision kicks ... like helicopter or back kicks ... splits aren't the optimal choice. Oh, you can make them helicopter OK ... I did, anyway ... but not as easily as a good, stiff blade fin. It's the wrong tool for that job. Back kicks ... well, I keep reading here on SB where people say they can do them in splits ... I never could, and I've yet to actually see someone do it. So I'm skeptical about the claim.
But expressing those limitations aren't a knock on the fin ... it's nothing more than a recognition that different styles are better for some things than others. That said, there's also a lot of misinformation about splits ... that they create silt, for example. Sure they do ... if you overkick them, which is easy to do in a fin that doesn't offer much resistance. But if you bend your knees to get the fin up off the bottom, and keep your fins inside the slipstream, you won't silt with them. I demonstrated this to a student wearing splits less than two weeks ago, in fact ... and in an area where a stay thought tends to produce silt.
What needs to happen in these conversations is that people need to stop taking things personally ... and people who have no experience with the equipment should just kick back and let those who do talk about it. But that's a lot of expectation for internet conversation ... so as with most things, it's useful to have several grains of salt handy when you engage in these discussions ...
... Bob (Grateful Diver)
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