For both pregnancies, I was managed by a perinatologist (MFM) until delivery. However, in the OR, it was an OB-GYN resident who performed the c-section (with the MFM assisting). The first pregnancy was initially managed by OB-GYN and there was a big difference. I didn't think she had a good handle on the situation and was much more lax with recommendations.
I think some MFM docs are trending toward purely managing pregnancies and leaving the procedures to the OB-GYNs. I think this is fine provided that the MFM is the one calling the shots. I had NSTs every other week from the 3rd trimeseter for my first and every week for the second (I was anxious). All of this was dictated by the MFM. They are specialized in this and know what to screen in their patients with diabetes, as well as what indices to follow to determine if something is wrong.
I like to think of it like this (and this is in rough terms - just for illustrative purposes): To build a bridge, you need both an engineer (MFM) and construction workers (OB-GYN). For safety, the bridge is designed by the engineer because that is their expertise and they have a specific skill-set to do so. However, when it comes time to build the bridge, the construction workers are a better choice. They are used to building/working with their hands and know how to do so gracefully with minimal problems. Certainly an engineer could work in the manual labor and the construction worker could design the bridge. Each may have even participated in the other process at one time. However, for the best outcome, it is optimal to let them do what they do best.
Likewise, many MFMs have narrowed their skillset to managing the medical aspects of complicated pregnancies. They know HOW to do the surgeries, but if they do them less frequently than an OB-GYN, they are not going to be as good. Similarly, the OB-GYN physicians do a LOT of surgeries. However, they have limited exposure to complicated pregnancies simply due to the variety of patients that they see each day.
I think using the MFM to delivery is fine and using OB-GYN for the surgery is great too. Make sure (and they should do this anyway) that they are on the same page.