Couple of years ago....I went to Helsinki and stayed at a fine hotel about 6 blocks from the train-station. I asked the door-guy....any areas I should avoid? He responded with one corner/end of the train-station....drug-dealer hang-out. Naturally, about the 3rd day....I was at the station and observed what I considered a fairly 'nice' and clean train-station....but in three minutes....it hit me....there was this 40 meter by 20 meter area....which had some shady-type characters (obviously dealers). It wasn't dangerous, but the door-guy was correct...it was just a place to walk-through....don't linger.
About 20 years ago....I would have said this over Frankfurt or Hamburg. Since then....you need to really plan your entry/exit....giving zero time to linger.
Mainz/Wiesbaden? Both stations have roaming police, but if you walk out the side-doors....you might bump into the drug-crews.
I would imagine if you listed the top forty train-stations of Germany.....they all have this problem now.
What pumped up the drug-scene? I'd say mostly migrants....mostly young males, who got into addictions, and no one really saw the trouble brewing...until it was too late. The buyers? Male, and female.....Germans and non-Germans.
How things will go in twenty years? At some point....a crack-down will occur.