My Lumia 950XL took the April 2018 update last night. I selected to Restart Now and the update applied. After it restarted, the phone was stuck on the blue Windows logo for an extended period of time. This isn't the first time I've experienced this issue with an update -- the last time it happened, I simply forced a restart by holding the power button and the migrating data steps proceeded. Not this time. Repeated forced restarts, including removing and replacing the battery had no effect on the outcome.
I decided to pull the 200 Gb MicroSD card holding my music, photos, and files and try yet again. This worked and the updated completed. I then powered the phone off and replaced the MicroSD and restarted with no ill effect.