I like soloing the low level stuff because it helps me sort of get a feel for some of the different maps, where possible escape paths are, and where the fuzz may attack from. Most of the time, since you can only carry one bag of loot at a time, I find myself getting pretty chewed up as I run from building to van and back again 3 - 6 times, depending on the mission. I suspect the flow of a heist is totally different when you have communication and a plan with multiple humans vs. the two AI gun wielding meat bags you take with you solo.
I would say most of the more coordinated and even moderately difficult heists are un-winnable with lower level characters. I'm only level 9 and I don't have SHEET for skillz yet, so I know I have a ton of growth ahead of me. Some of the upper skill level stuff sounds like it would really help solo runs, and I have heard of a lot of stealth solo runs on the hardest difficulty being possible, and apparently YouTube has the evidence to prove that.
Reading that how-to guide Rude posted made a bunch of things and innerworkings a lot more clear for me, so I'm chomping at the bit to experiment with some of the stuff like not attacking blue shirts, keeping hostages for as long as you can to delay cop zerg rushes, finishing quick to avoid police buildup, etc.
I think there's a lot to explore and I look forward to it!