There are ways to deal with bullet penetration, Wahoo.
As the two posts above me state: "Concealment is not cover", so you need to make sure you are not being seen. After all, what can't be seen can't be fired at, unless they want to give away their own position without having a clear target.
It doesnt mean things become unbalanced, it simply means you have to adjust your playing style and look at cover differently than you did before.
In RO:OST you could hide behind simple wooden objects, things which normally would be counted as concealment and not cover. Yet in ROOST they are all cover.
In RO:HOS it's more natural: you can KNOW that bullets are going to rip through the wood, and if you realise that you have been seen moving behind that crate, it's not safe to stop there. You could go prone instead of a crouching position (and in the case of no deathmessages being on) you could confuse the enemy into thinking they shot you while the bullet actually went over you, or you could keep running, find a hollow in the ground to crawl into, etc...
All it takes is a bit adjustment, i dont see at all how things would become unbalanced. It's something that can be predicted and therefor avoided.
I think you are overexaggerating a bit, you make it sound as if MG's could rip through an entire map without anything stopping the bullet, as if nothing would be in between you and the MG gunner. That's just not true, and only true in the cases of light materials like wood, perhaps a single-brick wall etc.
Concrete is much more likely to stop a bullet, or steel doors etc.
In the end, in the ROOST you could play in a different style, where you could be spotted and duck behind any kind of object to be safe. In ROHOS you can not.It doesnt mean it is less balanced, just that there is more to keep in mind.