At the prompting of a squadron mate, I've been experimenting with the Yak 9U (after a failed experiment with the Yak 9T, "T" as in "Try the U"). I've found that it can be quite the Mustang killer, and there's plenty of those flying about that finding prey is fairly easy.
There are a number of downsides:
(a) you have to get in real close to hit anything. When you think you're in range, get in closer. Make it a habit to hold your fire until you're less than 250 out.
(b) Small ammo load. You don't want any of your bullets to miss. See (a) above.
(c) ROF and trajectory seem pitiful. See (a) above.
(d) Anything shooting at you will knock out your oil pan. Once your motor starts to smoke, you better get down to land or climb up to bail.
(d) People think a Yak is an easy kill, so you'll frequently find bandits bypassing your wingmen's Spitfires and Mustangs just to jump on you for what they think will be an easy victory.
(e) Definitly not a plane you want to use for bomber interception. See (a) above. When you're that close, you make an easy target.
(f) You only get a pair of 12mm machine guns and a 20mm cannon, so you won't have the punch that most of your opponents will have, but its enough to get at least 4 Mustangs if you don't waste your ammo.
(g) Did I mention that you'll frequently get ganged?
(h) It lacks the romantic notions of warfare that accompany the more mainstream planes, like the Mustang, Spitfire, Bf109, Corsair, and Zero.
(i) You have to deal with Stalin's purges after every flight.
(j) Nobody seems to make a scale model of the Yak 9U.
On the other hand, there are a number of upsides:
(a) capable of catching a Mustang, particularly below 15k where most AH MA dogfights take place.
(b) good visibility all the way around.
(c) small size makes it easy for a Yak to hide in the blindspot of most other aircraft, including Mustangs.
(d) all 3 guns are in the nose, so convergence doesn't matter.
(e) the camo paint job lets it blend in with surrounding terrain (except maybe the AKDesert map).
(f) you can get an outrageous amount of perk points by landing just a few Mustang, Spitfire, N1K2, or LA-7 kills.
(g) seems to have good flight range between fill-ups.
(h) climbs pretty well, or at least better than the Corsair, Mustang, Hellcat, or P-47. In fact, overall, it handles pretty well. I haven't stalled a Yak 9U yet!
(i) there's no such thing as a Yak dweeb.