I’ve been going from Rotterdam to Kerkrade pretty regularly lately. Usually get off in Aachen and grab a local bus from there.
Train return would cost me around €130. FlixBus is about €30 return. That difference is massive, especially when you’re on a tight budget.
Honestly I also find it less stressful. No switching trains, no running across platforms. Just get on, headphones in, and chill for a few hours.
I’m using it while getting ready to move to my first place in Kerkrade, so I’ve done the trip a few times now. It’s kind of become part of the whole process for me.
If you’re traveling long distance in the Netherlands and trying to save money, it’s definitely worth checking.