They will have a harder time because they will effectively be a new outer border and especially for Bulgaria there are some challenges in dealing with this. Which is a pity because I personally would love Romania and Bulgaria to become part of Schengen. It makes life so much easier if you travel there frequently.
Honestly maybe getting Croatian borders dissolved will be enough because in-season traffic is just such a sad waste. I would guess that traffic on the new borders will not be as bad.