You see you can't consider the F-35's merits in isolation because it can't operate independently, certainly not in stealth mode, in a real war scenario.
The F-35 can't use its own radar to see what's around it in enemy territory. The radar emissions would give it away. So it depends on another much bigger aircraft, AWACS, a 70s vintage Boeing 707, to keep an eye on the battlefield and feed it information.
The F-35 also can't go very far in stealth mode relying on its limited internal fuel capacity. It can't haul drop tanks for extra range because they would show up on radar and defeat the stealth advantage. That leaves the F-35 totally dependent on aerial tanker aircraft and they have to be deployed well forward to give it juice on the way out.
So the Russians have done the math and they're very good at this sort of thing. They know that one (and it's only one) way to skin the F-35 cat is to go for the low hanging fruit - the tankers and AWACS control aircraft. They're bit and slow and ungainly and that makes them really easy meat. And those pesky Russians have built long-range missiles specifically for that job. Even if you can only take out the tankers, the F-35 is reduced to a one-mission air war. You don't have to shoot it down, it'll crash all on its own when it runs out of fuel.