One big reason why not to buy a Rqspberry Pi for Self Hosting is the price. If you want to run more heavy services (like jellyfin can be), you need the Pis with 8GB of RAM. Though those where already expensive before the sharp rise is RAM pricing. When you include a fitting power supply and a small cooler with fan (so that you don’t run into thermal throttle fast) you are probably already at like 110€ at least. For that money you can also get a used mini PC (those that companies use for their employees), that already has more power and offers better upgradebility (RAM, storage). Also for some services you might run into the problem, there is no version build for ARM devices (though that happens very rarely nowadays).

Though with the RAM pricing problem every computing device gets more expensive. Just include such mini PCs into your comparison to make a good decision.

Typically no. Though recently I had to get some neurological tests, including one, where you need to look at a changing pattern on a display, while electrodes check your nerves/brain activity. The assistent again and again told me to look harder. (Apparently they can see that in the graph of the nerve activity) And it is really difficult to look harder, so I stopped breathing while trying.