I've tried them once, and my experience with Flixbus was a disaster. They optimize for cheapness above everything else.
Their drivers can't do anything except scan your ticket and deny entrance if the computer says no. They don't speak any local language or English. Any minor problem cascades in delays of half a day. Their bus stops were druggie/thief zones where I felt really unsafe. After their train was 2 hours late, I was a minute late for my connection. The bus driver saw us running to his bus, smiled, waved at us, closed his door and drove away, showing his middle finger out of the window.
Their drivers can't do anything except scan your ticket and deny entrance if the computer says no. They don't speak any local language or English. Any minor problem cascades in delays of half a day. Their bus stops were druggie/thief zones where I felt really unsafe. After their train was 2 hours late, I was a minute late for my connection. The bus driver saw us running to his bus, smiled, waved at us, closed his door and drove away, showing his middle finger out of the window.