I've ridden both Flixbus and ICE extensively, had good and bad experiences with both and would say that they are very comparable in most aspects, except travel time. If I would find myself in a position with less money again, I would switch back to Flixbus (though the gap when you have a Bahncard 50 isn't even that big on popular routes).
I wouldn't be surprised if they would lose significant business in Germany when the new Munich-Berlin ICE track is opened at the end of the year. 6hrs (ICE) vs. 8hrs (bus) will suddenly become 4hrs vs. 8hrs.