I'm thinking of purchasing a H9 Max. But it is very importent for me, that the Harpeggiator runs easily in sync to a external MIDI-clock. I'm worried about all the threads about problems with that.

Have these problems been solved by FW-updates in the meantime, or…?

As far as I know we've resolved the issues with Harpeggiator. You might need to manually retrigger or use MIDI START to get the phase of the sequence correct with a song (obviously).  We explicitly fixed a number of MIDCLK bugs when we did Looper on H9 (as I stated above in 2017).  I remember specifically: 

– fixing a bug where we would occasionally drop a MIDICLK tick (every few minutes or so) due to a race condition in the memory access (there are supposed to be 24 per beat).  

– adding extensive statiscial filtering to the MIDICLK tick stream in recovering BPM 

– adding MIDICLK Start as retrigger for the Harpeggiator sequence.