Ost mortar > Soviet (even with flares).
In addition ost mortar doesn't require specific tech (19/20 you are building t1), is supported by the best mg in the game rather than the worst, has a decent core infantry squad that doesn't require another tech building to get etc etc.
Additionally suggesting "asymmetric balance" as a reason to nerf Soviet mortar with no intention of applying that to cons maxim etc is simply wrong (and biased).
Edit: I'm glad you decided to include the fact it requires constant muni investment to be somewhat useful and that flare can be quite buggy - oh no wait you didn't