I suspect that most of the issues suffered by 121's come from a combination of poor packaging design this time around, ie the foam cradle, and the pressure that postal services and couriers around the globe were under during the first "Pandemic Christmas" last year. I forget the actual figures but it was something like three times the volume of parcels sent relative to previous years, with only half the staff working.
Parcels were being drop-kicked around the planet, and no loco was going to survive every bounce, specially such a highly detailed one as the 121.
I had hoped to hear an announcement by now on spares for them, though, agree wholeheartedly there. From buying my own and from reading accounts online, I'd guess that about a third of the models released need replacement vaccum hoses, hand-rails or both.