I used Flixbus a few times in their early days of them operating routes within the UK (when they were charging £2 on many trips) and my experiences were pretty dire.
First trip: delays on the M1 made me miss my connecting Flix at London Victoria and because of the pretty much non existence of any way of contacting them in that moment, I was left in the dark about what to do to complete my journey to Cardiff, best I could do for help was make a post in a Flixbus Enthusiasts group on Facebook in the hope that somebody, anybody, could help - managed to get a few answers before my post was deleted with it being an enthusiasts group, after about an hour, my tickets automatically changed to the next journey about an hour later.
Second trip: return of the above from Cardiff, no issues to London Victoria but the London to Leeds leg was dire - first we had to pull into a service station on the M1 to rescue passengers from a broken down Flixbus service, so we ended up with a full coach during the height of covid when passenger capacities were limited for social distancing, so wasn't too impressed at having someone sat next to me with no mask for 4 hours on that journey - additionally due to an incident on the M1, it took an extra 2 hours to reach Leeds.
Third trip: got to Leeds, awaited my service to Birmingham... no coach appeared, leaving us all to figure out what on earth was going on, turned out a contractor was removed or chose to remove themselves from the network which led to cancelled trips, no notice from Flix whatsoever prior to this trip including on the day of travel, we were all having to hammer their customer services for any info, eventually one of us got told the service we were booked on no longer operated - so I had to spend about an hour attempting to figure out what the next steps were going to be, this was an infuriating process contacting them through their website as if you left the page, the chat would close and you'd have to start from the beginning again - I eventually made my way to Birmingham, 3-4 hours later than originally booked.
Fourth trip: York to London with Whippet Coaches I think it was? Had to sit at Foss Park for about an hour waiting on a driver to take over for the remainder of the trip to London, no reason was given for the delay, don't think that driver said anything for the rest of our journey to London.
The frequent rate in which they make changes to services and timetables also didn't give me much confidence either, at least half of those bookings above changed at some point between booking and the date of travel, sometimes your journeys would change by several hours.
Part of me wants to sample some of their offerings again to see what it's like now and check out some of their coaches on the network, but these past examples puts me off that idea, I'd sooner pay the extra money for the speed and convince of a train.