M:S certainly has its quirks and limitations, it can be difficult to adapt to your will. But if you go with its workflow it can be rewarding and even surprising at times.
The 120 point limit is confusing to me too, I think maybe it has to do with MIDI ticks so it’s a value divisible by 6.
It’s annoying to have to do, but if you spend a bit of time making some custom 120 slice chains, these are great fun to mess around with. I found I have at least a few uses out of each chain I make.
The OctaChainer software makes it simple to make accurate chains, thread about it is here.