A lot of stuff runs great on SBCs, it's just that they're not as smooth to manage as a Proxmox server running containers or VMs. You also need several SBCs to reach the scale of what many do here on selfhosted and once you reach 4+ SBCs the old x86 server starts looking cost effective all of a sudden. The biggest benefit though is the no noise and very low power consumption, which is great for stuff that will be powered on 24/7/365.
Really a mix is ideal, so you can get the benefits of cheap running costs of SBCs and the power and versatility of x86 for the tasks that require it.