I've had Vudu for about 4 years and I have about 200 movies and TV shows in my collection. I've rarely had any issues that stemmed directly from Vudu itself; most problems were due to my Charter Internet service. I knew it wasn't Vudu because when Vudu would fail I would run speedtest to find that my connection was stalling. A modem and router reset would fix it, but for a while Charter was hit/miss especially at night times. The last 2 years they have fixed things so I no longer have any buffering issues. I'm located in San Francisco Bay area, not sure if physical location impacts streaming quality.
For all the bad reviews pointing the finger to Vudu for your buffering issues, make sure to test your internet connection to be sure where the problem is. I'm loving Vudu and I can't wait for them to stream 4k content. They're already on board with Dolby Atmos so they're keeping up with the times.