There are two factors that cause the problems mentioned here.
A lot of sharp operators are attracted by the "romantic" and "swashbuckling" aspect of flying. They end up involved in a lot of aviation businesses, not necessarily in the ownership end (because they tend to be skint themselves) but in the operations end.
Today, most people that get into PPL flying can't really afford it. This plays into the hands of those above.
I never bought into a group but looked at a fair number of them before eventually buying my own. It should be obvious to anyone with enough of a brain to pass the PPL exams that the costs will not be predictable and I don't really think anybody has an issue with that.
The issue is that somebody selling their share (which you are buying) is very unlikely to be telling the whole story as to why.
I think the best arrangement would be to get together with a few people one knows and trusts, none of whom are exactly skint, and buy a brand new plane between you.
The next best thing is to buy into an existing group, comprising of people one knows and trusts.
After that, one is into the territory where people sell shares because of e.g. disagreements on maintenance, or there is a huge bill coming up which the engineer has mentioned to somebody informally.