All 3 bases constantly send out swarms of units. Among them are Yuri Clones which are a massive pain in the ass and basically double your unit requirement, which is high to begin with since you'll be fighting against massive forces near lots of tesla coils. Just breaching the 1st base opening is a massive pain.
The mission reveals the Kremlin, gives you Kirovs and has a special trigger when building a Nuke for a reason. It's specifically designed not to be beatable through ground, having you frantically defend your base against an onslaught until you get through by air or nukes.