Well, dodgy moldings, detail and fit. Combat being a posterchild of worst-case scenario vac kits. "Combat - It's not just a brand, it's a description!" Yes there are high end vac kits like Koster and Dynavector, but they still need more fettling and DIY engineering solutions.
Back in the day (1970s) the other challenge was obtaining detail parts like props, engines and wheels. But that is no longer the case, with a dizzying array of aftermarket bits coming out monthly.
I think today the main reason most people don't want to try a vac is because there are so many esoteric subjects that have become available in injection plastic.
I think every single vac kit I ever owned (not built) has seen an injection molded replacement appear. The exceptions are the Wings X-24A/B and the Nova C-141A/B. Heck, the Wings M2F2 (Steve Austin: "I can't hold her, she's breaking up, she's breaking up!") is soon to have a plastic competitor.
That being said, if someone offered me a Dynavector TSR.2, I'd jump on it - the Airfix kit is a monstrosity with weird shapes and blobby detail.