I’ve worked on two 5100s, first one was a horror story that STS had rejected as beyond economic repair, the second was far more straightforward.....but it brought back bad memories!
Back in the 70s, when these movements were designed, the use of plastics seemed like a good idea and probably enabled the movement to be built cheaply in quantity. Fast forward to today, history doesn’t judge them favourably, but they were never intended to last 40+ years!
In my opinion there are two major weaknesses with this movement.........but I’m not really a fan of chronographs if I’m honest.
If I had to pick a favourite chronograph the Omega 861/1861 would get my vote. Re-launching the 321 makes no sense to me.