Thumb fretting is an ace accomplishment (ask a Balalaika player ) and you shouldn't really judge ...the individual concerned may have a pain or joint problem.........I fret with a thumb now and again because of wrist pain.... and because I can play a balalaika (not well)
Sometimes playing out of tempo or off beat in a group environment is down to not being familiar with the piece as played because you are new to the group...come on we all have to learn ...telling someone to sit out and learn by tapping your uke would get a very definite reaction from me ..the tap would be somewhere un-expected but deeply appropriate ....
(I am joking, before I get accused of losing the Ahalo spirit)
I just joined my first ever Uke group...I love it ...I am the "newbie" to a group environment and I have to hold back with the strumming.....
A) because most of the group play with plectrums , you know those felt plectrum thingies...I could not care less......it is their group and I feel privileged to be accepted and allowed to join in .......
B) I am probably the longest playing member of that group but again A) applies so softly I sit and strum with my fingers....and learn the songs of the group and how they play them ...and have super fun...
C) I do not see why ukes cannot be played with plectrums..they can sound brilliant and mandolin like ....and I have in fact experimented with a mandolin and retuned it as a Low G uke ...only to discover why it always sounded s** as a mandolin ...because it is a cheap Ozark and E strings (uke tuning A string mando)goes from open E to F # on the first fret two semitones .....so I presume the nut slot is too high ...(Luthiers --help ?)
D) If someone is doing something wrong or daft or annoying ...well tell them...however I get the impression that people just want to be vile and hurtful and enjoy ripping people apart ...just because they can...tell people gently ..and pleasantly and nicely and constructively....then their reaction dictates the way forward ...Lizzy , my wife has joined our group ( oooh "our group" already) as the tambour-egger-iner (not having a dig Olde Pharte) ..the first thing she did was explain that she had no intention of playing or learning to play a ukey-lay lay
but would like to sing and do a bit of percussion (as a complete novice ) if that was okay and please to say if it was or was not Okay...........so far it is okay ..but she always asks for and gets advice and feedback (in fact feedback on an acoustic tambourine takes some doing and has to be applauded )....
Sorry ...is long ...like my cat.