Grounding a newly release airliner is not new. It's only news because of the two crashes and 300 victims. I'm pretty sure these airlines are careful and still have some of the older 737/A320 that these MAX were supposed to replace.

Issues with new airliners and engines -specially engines- are not rare at all. So this sort of grounding is not rare: see 787 first grounding (battery issues), current partial grounding (Rolls-Royce Trent 1000 issues), or Pratt & Whitney ongoing issues with the A320NEO engine (PW1000G issues).

You can imagine the sort of clusterfuck when PW needs to produce 60 engines a month for Airbus, only manages to build 20 in its factory... but need to send these 20 engines to replace already delivered engines because they are defective. The situation has improved a lot lately, but 2017/2018 were tough...

Only the A350 introduction has so far been relatively event free. Probably because the Trent-XWB is a derived from the Trent 1000, so the 787 took care of the teething...