Low salary: if you really find it annoying to see this in every single review of FlixBus, this is not a joke! On other hand, your net compensation will be!
DO NOT (!) trust HR or your future manager on the interview stage as they would probably tell you the salary will increase over time. It's simply not true: many whom I knew from FlixBus left the company after 2,5+ years without any pay raise whatsoever. In case you would still like to for accepting the offer, try to negotiate higher salary from the very beginning. If you do accept the low ball, I wish you good luck surviving in the most expensive city in Germany. Please note, their salaries are up to 30% lower than Munich average.
No career progression: don't anticipate to have a promotion unless you are belonging to one of these groups: tech (data scientists or SWEs), extremely above-the-average overperformer (you put your entire life in your job and work 50h a week), friend or a buddy of your manager (there's a 'cool gang' elite of veteran workers who get all the perks). Expect to lose any motivation after 1.5 years of service, there is no recognition for your or someone else's accomplishments. The promotion sequence is blurry, unclear and unfair. Your dreams of having been promoted one day may be shattered yet by another newly-appointed mid-manager who would say one another 'prove yourself'
Poor internal infrastructure and chaotic processes: as you might guess having so many unmotivated, low-morale employees results in an atrocious level of execution of the most basic tasks. Some of submitted requests or asks could take weeks to be resolved. The execution of all kind of things is extremely poor and done in a negligent way. Granular data is often only available through SQL queries as there is a major lack of dashboards and tools (because of cost cutting of course). There is no foreseeable fix for these issue as nobody takes any grade of responsibility or ownership on their mistakes or incompetence.
Awful middle-lever managers: (this actually depends on pure luck - there are fairly good and bad managers in the mix). I was unlucky to be a part of a team with a person who lacked any empathy or interpersonal skills of any sort. The one acted purely using his status without paying attention of their employees' needs making even the most basic interaction into the awkward and stressful experience. Be ready to face some out-of-space highly-demanding micromanagers and control freaks.
Greenwashing: company management desperately tries to play the 'we are green' game to appeal to the younger, more environmentally conscious audiences. That's however is ridiculous having in mind they operate extremely eco unfriendly diesel buses. It's funny to see the ads stating 'Flixtrain fährt jetzt mit 100% Ökostrom' while knowing FlixTrain still consumes electricity owned by DB Netz AG and there is literally no way anyone can trace the source of electricity (let it even be Green Planet Energy or Naturstrom AG), DB does the same.