I have heard of operators who would not accept the PADI self reliant card (and others that would not accept any card). I've been diving solo since my first dive in the winter of 1961-62. I've been told I could teach the solo course (but I'm not an instructor nor desire to be). However, when I am diving a new location I generally prefer to be buddied up. Here at home I solo at least 95% of the time.
My primary concern with anyone undertaking solo diving is whether they know their response to an unexpected emergency situation. If one panics, I don't recommend solo diving at all. In the 50+ years I'e been diving, I've had several incidents where panic might have killed me, yet I remained calm and followed my training/experience.