Oldschool SFF, LPX, proprietary and cramped form factors are not so "scary" any more. There is no worries about if/how you can upgrade them in the future, because that future has already happened as far as their potential goes. You are not "stuck" with it as a large investment sole PC that you need to get a few years out of. Many tips and wrinkles, hacks and mods have been figured out in the intervening years. Modern storage is massive, you don't need to worry about adding another 3.5 HDD year on year as your storage needs expand, so don't really care about internal drive bays beyond one or two. Worried you'll need the full 19" slot length to expand the graphics at all? It didn't really happen, there was always shorter cards, unless you wanted top of the range monster, but you know what, the half the size midrange from following gen does just as well.
Anyway, they won't suit the people who wanna get 5 soundcards in one box or things like that, but for building a set configuration, they are fine, even desirable because they often take up half the space of an AT or ATX standard lump. It's also more fun with the artificial constraints to try to "build them out to the limit" to see what the most you could do with it was, you have to get more inventive, creative... devious.
I would not recommend them for beginner retro machines, by choice, better to learn the ropes on a "standard" XT, AT or ATX format and cased machine or two. However, should you have your old one, or get given one, or find one at pocket change pricing, then while they might lead to extra bumps in the learning curve, they can still be worth it. Well known brands usually have enough information online these days to prevent extreme frustration in getting them to do anything.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.
