IMO what is underrated is not the lethality, but the performance capabilities. You can see it doesn't turn too much worse than a spitfire (no mean feat!), and its climb rate and acceleration are good. It's a superb diver, can pull some Gs, it can latch onto a target and stay there.
And it's fast. Much faster than I think most folks realize. This is where it is mostly underestimated.
It starts out a bit slower on the deck, but going up just a few thousand feed above sea level and now it's right up there with P-51Ds and 190D9s.
http://www.gonzoville.com/ahcharts/index.php?p1=190d9&p2=p51d&p3=yak9t&p4=yak9uThat is the main thing I think folks don't know when they try to run away from one. They keep thinking "I can make it, it's just a yak!" and then instead of making a turn back towards the enemy while you still have room to manuver, the yak closes in on your tail and you now cannot shake the plane that turns not-much-worse-than-a-spit. So it's the inaction due to not realizing how fast it is that leads to them flying straight and letting it close the gap, at least IMO and from what I've seen.
EDIT: The speed issue is bigger also because the Yak9U has constant power, whereas the others have 5 or 10 minutes then have to cool down. At that point the yak is actually faster than the P-51D and 190D9, because it has no WEP to cool down.