I've never had any issues or delays DRSing from TDA. They have a pdf form I send in through the message portal on their website. A few days later, the transfer is complete with no questions asked.
I briefly tried out Fidelity for a Roth IRA but didn't like the user interface. Think or Swim on TDA seems to have a lot more details/features. Also, I didn't like how Fidelity's app was constantly logging me out. On Think or Swim, I log in once per day which makes it convenient for using my phone as a live ticker viewer on a wireless charger at work.