Might be interesting to flip the problem around.
Rather than beginning with: what equipment? Instead: what problems?
From that standpoint, there are only a few risks that differ from buddy diving (if they even do)
Gas loss, entanglement, strandings, and planning/decisions?
When you unexpectedly find you can't breathe while on a dive, you need an immediate fix. Anyone who's had that problem in real life knows. It is not the same as any drill or skill in any course. Forget about that "breath hold swim" or "controlled emergency ascent" that you think you can do. Especially if you're sinking, in a current, carrying a camera, or having any other problem simultaneously. Have you seen how much of a joke the PADI CESA drill is? SCUBA isn't freediving.
A tec-style side-clipped Al40 with its own reg is so easy to manage and include!
People whose plan is "I'll just swim for it" usually don't make it to the surface alive or unharmed.
But this doesn't only apply to solo divers!