P-39 had excellent low altitude performance but was terrible at high altitude. Since most air combat on the Eastern Front was at low - medium altitude, the Soviets were fine with the P-39's operational limitations.
My recollection is the Soviets liked the P-39 as a hard hitting ground attack aircraft that could fight its way out of trouble unlike the Il-2.
P-39 wasn't used for ground attack by the Soviets.
They actually liked it because it was the only Lend Lease fighter that had it's armament in the nose (37 or 20mm cannon and 2 .50 machine guns) .30 wing guns were typically removed.
Soviet crews actually loathed the Spitfire, oddly enough because of it's wing mounted guns.