The main issue plaguing P-27 family is the lift and Gs factors, along with some guidance bugs (mostly chaff related).
Basically the missile is dead when it reaches speeds below M1.5, and is unable to maneuver to intercept even though its big fins and body should in theory provide for much more lift/vectoring than other comparable missiles (of course at the expense of more drag). For example, in contrast a 120 can still climb and hit a target even when it does M0.4!
In addition it appears that P-27s are unable to pull more than 10Gs in maneuvering whilst commonly available data indicates they should be easily able to do more than twice of that number? A pheonix does better than ER pulling Gs!
Those two factors alone reduce the kinematic effectiveness of the missile by a large percentage, IMO.
Then there is the chaff bug, and some weird fuse behaviors when they can literally flyby someones canopy without detonation... I see people dodging R-27 by just doing retarted aileron rolls head on, from NEZ. Missile flies right by, no fuse... kind of pathetic really. These last two issues could easily be fixed by amending scripted .LUA values.