Disk space. Ram. Probably the two* main components of a server that will make running Cloudron (or any software, really) a pleasure or a pain. But I think the easy answer isn't necessarily the best - buy a server with lots of disk space and lots of RAM - because use-case factors in alot. Perhaps some sort of pop-up questionnaire at some point in the purchase stream or set up process asking which apps are likely to be used would be one way to give a user a heads-up that their 4GB RAM, 60GB SSD server isn't going to cut it (or in this case an 8GB running Gitlab! Plus, it seems they are running additional software on that server, this "Mesa de Ayuda" which seems to be some kind of helpdesk software... Cloudron clearly advises running only Cloudron on a server and nothing else, so that was utilizing the same minimal 8GB of RAM).
That said, I moved my Mastodon app off of my main server which has 64GB RAM and successfully run it on another server with just 4GB RAM plus 4GB swap, plus a Wordpress app, with stable results. I've also run a Presearch node on a 3.5GB RAM server along with Cloudron, and it all worked.
*A third would be using Cloudflare. I know why ppl use it, I just don't know why ppl use it.