It's my opinion, and seems to also be the opinion of the devs, that it's preferable to try to work within the confines of the original Red Alert source material rather than adding completely new units which would pretty much be a last resort.
According to this thread, the greatest balance issue seems to be the Soviets lack of AA capabilities at sea to properly counter Allied helicopter air support..
The MiG is a unit that is already in the game.. MiGs can fly over the water and support the navy... (just as helicopters do). Give the MiG AA capabilities and it solves this issue.. The MiGs can counter the helicopters. This makes logical sense, doesn't require the introduction of an out-of place unit that wasn't in the original game, and has been something people have asked for over the years. Obviously there would be other effects it would have on balance that need to be considered, but to me it seems to be a reasonable approach to addressing this issue.