> I've have personally never seen a good example where a cross vendor account provisioning actually working.

That's not what this is though, is it? In other words, isn't the (anti-)pattern you describe an argument in favor of agents setting up your accounts instead?

You can tell your agent to buy the domain at registrar x, manage DNS at y (and maybe configure DDoS protection and CDN), and host your content at z, and if the agent is good enough, you don't even need to understand the details.

You end up with individual credentials for each service, rather than a web of account relationships managed by a single "portal" SaaS.