Doesn't mean you can't put them on petrol motors, doesn't stop people from putting Greddy turbos on their cars
The Holset wheel technology is argueably better than Garrett equivalents, they are all plain bearing turbos keeping the price down - HOWEVER some argue that they can near compete with the BB core Garretts in terms of spool due to the really efficient wheels. They have a decent solid shaft etc, and can handle lots of boost.
I have mates with them but (alas so common with projects) none of the cars are tuned yet. There is a L28ET with a HX40, a 1JZGTE with a HX35 and a 2JZGTE with an HX50 I've been waiting for some time to see how they come out running in real terms haha.