Well, the possible ED issues for one. But you're probably right in that considering the amount of use, it probably is rare. But I've seen many reports regarding that on the forums. And when/if people do get sides from Fina, they seem to stuck around, even after discontinuing the product. The whole mechanism of action sounds unappealing, preventing/lowering DHT conversion. The rise in test is due to Fina preventing test to be converted to DHT, and so I think even though it can look good on paper, the net effect of slightly increasing test and considerably lowering DHT to be negative in many ways, like mood, energy, libido etc.
But I don't really know too much about it to be honest. I've only read about it and about peoples experiences with it. It just has always struck me as something I'd want to stay clear of. I also don't suffer from hair loss, so I guess I can't fully relate to the burden of losing your hair. And I used Accutane for 6 months as a teenager with no issues, so there's that also.