If your like me and have made your bones, and bucks, the risk is non existant. Necessity being the mother of invention, I forge ahead alone, though I often wish I had a partner. I have heard the comments by VC that a single founder model usually fails. I think that is valid. In my circle, I don't know any other single founders, so subjectively speaking it seems that the VC guys are right.
I believe it takes a psychologically unique individual to go it alone. We are as a species evolved to work best in a tribe. Those who stand apart from the tribe and are successful are few and far between.
My list of downsides are:
You make every decision alone.
By definition there is no synergy in a singularity.
VC perception is against you. If your not self funded, your going to have a hard slog finding funds from this group. Keep in mind that poltically speaking, manipulating founders is part and parcel of the VC game. A game made more difficult in a single founder model.
During the early stage, pre employee, you have to be a bright and lucky person to make every right decision on your own. You will make more mistakes.
You better be disciplined. If not your going to fail alone.
The upside you ask?
Only one that I can think of and that is that at the end of the day, since the system is so stacked against you, if you make it, you own it. And that has tremendous rewards. For me it meant very very early exit from traditional work patterns. CHOICE, CHOICE, and more CHOICE about every aspect of what I do. No one interferes in any way when you pay your own way. But, the old saying that its lonely at the top, is no lie. If you can handle that, the rewards are worth it.