I don't know either , it's just one of those things. A lot of people say the mig 21 runs poorly for them compared to other aircraft but in my game it runs fine. I don't know why.
Anyway the cockpit stuttering when looking up is only a very small thing , the biggest thing is the no radio functionality and the flight model.
I can understand why it has SFM , they said they would release AFM later and i was well aware when i bought this , but it's the other bugs that i wasn't expecting that impacted my enjoyment.
I'm sure it will be good once it's finished , but right now i can't recommend it.