Within my rec quals, narcosis is the big limit in my mind. I had a dive buddy wandering narced into the depths who might not be alive except for me.
Locally, you need not be deep, so my solo has been above 50'. I was making plans to explore to 85', which I think might be it solo for me without a trace of helium. I know I've been confused by my dive comp at 100', so a solid buddy as a spare brain would be a good hedge as narcosis gets more pronounced. I dive sidemount doubles. I think 100' might be my limit with a solid buddy, for narcosis reasons.
My other limit is 'not much new'. Gradual progression and exploration. So a new site I explore gradually out over several dives, depending on conditions. Not overextending. Being fairly certain I have a good reserve, in general not merely gas, to handle unknowns.
In terms of with or without a buddy, I wanted to show a solid buddy a site I'd not dove yet, and so dove it first solo to be sure I understood it before taking him there. That is a bit more that I'm the more experienced than buddy/solo difference though. I had a chance to dive it beforehand, and so felt better knowing it for going with him. Yet, I would not take several merely ok divers I know to a new site I had not yet dove, and would likely be fine exploring that site solo first, given a full reading of dive reports on it and favorable conditions.
Exit conditions are also a concern, so I am conservative on conditions there.
I also have to be rested and not wiped out from prior dives. We had ideal dive conditions a bit ago, my gear was all organized and I drove two hours down to Monterey. But I was not really feeling it. When I got there, I used the restroom, turned around and drove home without even looking at the dive site.
I've not been night diving in a while, but have no issue venturing into that solo in known waters. Beyond thoughts of what creatures of the deep might come up to feed in the dark. Muahaha... I've thought a predawn or dusk dive would be interesting to see the underwater transition. I'd rather do that solo before full night as it has been a while. Unknown waters at night would give me pause solo, unless they are known as rather benign.