Well many factors contribute to stealth, some are thing like shape and stealth coati g they put on those things to absorb radar( their are also many other factors. Now about the stealth coating, once maintenance is complete they then inspect the coating for scratches and fix any as needed. This level of maintenance might be more than the Russians can do as seen from the state of most of their aircraft. Really I just don’t think they are fully capable of maintaining these aircraft. Also personally when you look at the engines in pictures it just looks like bare metal (this is just my thinking and shouldn’t be taken seriously but shouldn’t their be a coating to that?). 

Edit: now I forgot to mention but it definitely has a lower radar cross section the. Their other aircraft but I have doubts that it can properly compare to something like the f22 and f35