What is the problem you see? I am interested why i should be cautious. It seems like offline app that uses .md files. And the authors seem legit bootstrappers with quality projects behind them.
The free license and the one-time purchase license do not allow for commercial use. You would need to purchase their Enterprise license to use this commercially. Is it expensive? Not really, $50 a user per year isn't necessarily expensive when considering price alone but when compared with other alternatives in the space I'd rather not deal with a yearly license. I used Obsidian Beta up until they released their licensing.