There are plenty of failure reports on the rc390 forums but those guys race those bikes hard. The first thing to fail with track use is usually the head gasket. Otherwise, some overheating issues.
Haven't heard anything about the cam tensioner and cam chain.
From what I know, those KTMs are less reliable than Japanese budget bikes (Ninja 300/400, Yamaha R3) but that's more of a testament to the competition's quality, rather than the lack of quality of the KTMs. Ninjas especially are known to be hard to kill, even with track use.
On the flip side, those Japanese bikes are all mellow and tame compared to a Duke. I've ridden them all, Duke 390 feels fast and fun compared to the others - that's coming from a guy who traded in a GSX-R 750 for one.
For me, the Duke is a second bike - a toy. I'm fine with it breaking (if it ever does, no problems yet) since I have a reliable Japanese main bike. If I were to only have one motorcycle, I'd probably go for something else.
2019 390 Duke