Well, according to Home Assistant Analytics there are currently 490 people using KNX with HA. I’m one of them (or 2 maybe, not sure if my dev environment reports there too ) and until now very happy with my choice for the last ~4 years.
Not sure if I second your opinion on its expensiveness - it is a quite big initial invest but then you can add good quality devices for moderate money. It heavily depends on the size and features of the desired installation.
Forget it if you are not building new or completely renovating your electrical installation.