I started with guppies because I'd known them by their old reputation for being super hardy, and great beginner fish. Had I known then what I know now, I never would have started with them. Or at least, I might have sprang for a trio of fancy, hobbyist bred guppies rather than store bought mutt guppies. I had way too many losses, and too many experienced people guiding me who couldn't find a flaw in my husbandry to account for the losses, and warned me how poorly bred they are now. When you discover how in bred and mass produced they are, and how often they're infected with round and flat worms long before they'd made it to a store, it makes you realise the impact poor breeding has, and even the best husbandry can't make up for it.
I've had a better track record with keeping otocinclus - wild caught and famously delicate and sensitive- than I have with shop bought guppies, and that's even with keeping the otos in harder water than they should have for the first nine months I had them. When a wild caught, delicate species that was likely caught by stunning via cyanide is hardier than a pet store guppy, there's a real problem.