@EW800 A bit of a late response, but I had a loan sold to Flagstar in 2018 and they were fine; one of the better mortgage servicer websites I've used. One con is that it took about 2 months after my old loan was reported closed and paid before the Flagship one reported (with original amount and date from previous loan thankfully), so I took a temporary score hit.
The only other con I can think of is that like with almost any other servicer, they can and do sell mortgages to other servicers and about 2 years later I ended up with Freedom Mortgage, which was absolutely horrible. I have a VA loan which is eligible for IRRRL refinancing program at any time. They would constantly call me to refinance for a rate .125% or .250% lower (with the accompanying fees rolled into the new loan of course) and one very shady rep after hearing "no" from me, called Player 2 and told her he had just spoken to me and I had agreed to refinance and it would lower our monthly payment (true, but also would have required us to pay that slightly lower payment for another 6 years), and he just needed to get her signature on the application. 
I immediately filed a complaint and refinanced with NFCU within days. But again, that was Freedom Mortgage that they sold the loan to, and I had zero issues with any aspect of how Flagstar handled the account.
