Just my personal opinion but I feel ‘unless’ carries a sence of negation and subjunctive mood carries it too. Double negation or multiple negation only makes things difficult even if theoretically it’s possible for a sentence. At least it’s better to avoid such an expression.
To me ‘unless’ has an air of gravitas about it since it often indicates a particular and very special condition. This is probably because it was used in that way in the past. It pops up in the 17th century English translation of the Bible and is used there in the subjunctive. I can’t quote ad lib from the Bible but this could be the sort of thing: Unless a sinner repent, he will face damnation. In conclusion I would say it’s perfectly acceptable to use it in its ‘if …not’ mode if there is a seriousness in its negative condition.
“Because I wouldn’t feel that I was speaking to the intelligence community unless I had good reason to do so.”
" I got support from nowhere except the gay community. The gay community taught me how to survive HIV disease, and I don’t think I’d be here today unless I hadn’t reached out to them and built a bridge."
Remember the great high school cross-country star?, he wrote, I did it only to prove myself to Dad, running until dizzy, stopping only to vomit by the side of the road. It got to the point that I knew I hadn’t run well unless I had thrown up.