One of the main problems with those 503 errors are that appeared almost randomly. So, every time I had to contact support, they asked me for steps to reproduce. And that was not possible. I don’t say that it was their fault. If you cannot reproduce something, you cannot investigate it.
I agree that 503 might happened for many, many reasons. I had in the past due to SQL injections attacks. Or xmlrpc.php attacks. But all those were fixable. Varnish was something I (or support) never managed to fix. So, I just used to have it disabled.