I used to cave dive, started in around 1971 cave diving. Met some of the old timers, well they were not old timers then. Then I met she who is most important in my life and it was told unto me that thou shalt not cave dive any more!!!!! Now, she allows me to solo dive, deep dive, fly airplanes upside down, race motorcycles and blow stuff up, solo hike Montana grizzly country but no cave diving! This should give you some idea of where she places the danger levels of various things. I tend to agree, cave diving is a dangereous sport. It is a sport. It ranks right up there or higher than just about anything in danger and required skills. It requires great skill, proper equipment, attitude and training, physical conditioning, preperation, planning and even then cave divers die. It is a risky undertaking and the main reason is the overhead environment plus total darkness.
I, on the other hand, rank solo diving as only slightly more dangerous than buddy diving. I am not going to argue about good buddies vs bad buddies because this is the solo forum anyway. That is how I rank it. As you stated there are many variables and I am sure there are some dives that done solo are very dangerous but still, over all, I just don't see solo diving as dangerous beyond the scope of what could be encountered in any dive, buddy or solo. We have discussed redundancy before and down to about 60 feet I consider the surface my reducndant air supply. Deeper I shift to a double set, H/Y valve set or similar arrangement depending upon what I got at the moment. Since I dive quality vintage gear I rebuilt and maintain myself I do not worry so much about mechanical malfunction like you might diving modern plastic stuff maintained by the LDS geek that just installed nitrous on his Civic and ground effects with drywall screws. OW solo diving has the same environment, conditions, dangers that buddy diving the same dive site would have. These are readily defineable. Before any dive you should study the site and plan for it, buddy or solo, and make appropriate plans and gear selections to support that dive.
I just see very little common ground between OW Solo and cave diving, they are very different animals and solo cave diviing, please put me on your insurance, thanks. N