PvP or PvE, the Soviets have a very well-rounded army (+commanders) without any major drawbacks or weaknesses. So yes, they are hard to fight.
How to fight them is too much to write down - I suggest watching popular YouTube commentators casting replays of high level games. Best channels are: AECoH, Stormless, Tightrope, GeneralsGentlemen and The Propagandacast.
For example, you'll see the top players escorting their vehicles with infantry or support weapons as much as possible so enemy infantry doesn't get a chance to get close enough to throw AT grenades.
I'm probably not aggressive enough against the CPU. I should probably work on boxing him into his town. There mortar sucks and their MG isn't great either, but the Penals more than make up for it.