I guess I would need to know what the entire route was.
In Germany, the Flixbus was coming from Munich, stopping in Nuremberg, where we were, and then on to Pilsen, CZ, where we wanted to go (and the bus went on to Prague). Well, traffic, delays, the bus did show up, but about 3 hours late.
Similar experience in Brussels, bus started I don't know where, going to Utrecht, bus never showed up, eventually we ate the tickets and took the train.
In both cases, I found their information on status severely lacking on the app, they usually do not have staffed desks, or even bus stations, just a "stop" by the side of the road. So support in the event of a delay can be lacking.
Basically, I would feel better if the route was more direct (starting in Bath would be ideal) and I knew the traffic would likely be manageable. Even with National Express, I have had issues with traffic in London adding 30 minutes to a bus ride. At least you will not be going through London.
That said, yes, I have also had great experiences with Flixbus, and would consider them again.