Castle will be devastated.
Make all the barriers that lead to outside indestructible until the round actually starts. Problem solved.
I just think it ruins the game because the main focus is on breaching and room to room combat and running around outside and shooting through tiny holes at the spawn isn't in the spirit of the game. Another issue with combating it is that on the most common areas, there's so many options for defenders to bust open a wall and get a couple kills that it's way easier for them to get the cheeky kill than it is for defenders to predict/spot out the attack and counter it.
Pixel peaks are hard to beat too given how guns work in this game. Basically, if you're holding a tight angle and peaking one pixel, you'll see them just barely and can hit them but everyone else will see you hitting the window frame or door frame. Which results in no tracers being seen by the other side.
It's an issue of the person shooting basically seeing the bullets going out of their scope, vs everyone else seeing them coming from their gun. Ubisoft could change that, but then people would get frustrated over their bullets hitting frames when the scope shows them peaking outside. They could make some kind of indicator show up on the speaker's screen showing that their shots are being blocked, possibly.