The lumbar and sacral spine are the ones affected on rotational, side to side, and forward-backward flexing motions. Fusing much of the spine segement in these levels can indeed cause significant mobility restrictions (I believe a decline of 30% or more).
For some patients, a brace or restrictive device is needed to facilitate the healing of the area. I would also expect that there can be some intensification of the pain immediately and after a few weeks of surgery while the wound and the surgery is healing properly.
Make sure that you are well informed of the risks and success rate of the procedure, and this can be discussed in depth with the surgeon.
Regards and God bless.