I had two different EX500's that I brought back from the dead on flipper projects. The second, ate it's head/cams up, creating a heck of a mess. I bought a used head on ebay, but that was bad too. Bought another, better, but half of the rockers and one cam were damaged with uneven wear. Long story short, I had a heck of a time finding decent top end parts to complete the build. Kawasaki clearly had issues with these engines, although when running well, they were a joy to ride.
I had one of the UK versions of the 500 water-cooled twins, and it lunched itself as well. However, I didn't maintain that engine properly, and they do require regular valve clearance checks and adjustment. Over here in the UK, parts for that engine are plentiful. However, those bikes tend to get run into the ground, because they're considered neither rare nor desirable, but rather workhorse bikes, and what disincentivises maintenance is the hassle of pulling out and then reinstalling the cooling system. Definitely, they're far from indestructible. For a lot of old Kawasakis (obviously, not for things like GPZ305), you can get new top end parts - even pistons and rings - from Japanese companies that still manufacture them, e.g. cruzinimage. While they're still available for the 440s (except for the rockers), the 400s and 440s will suffer when these parts are simply unobtainable or ridiculously expensive. I've thought about what will happen to my 440 when it kicks the bucket, and all roads lead to having a spare engine to hand. Spare engines are great, but still, they have some wear and they're getting rare. The 500s are far from rare around these parts.