Bro, it has nothing to do with the stability.. core UI is basic version of one UI..in core UI you won't get all the features of one UI..but you will get the core features of one UI like dark mode,new camera UI, digital wellbeing, focus mode, new device care and so on..
As M20 and M30 got core UI, so there is maximum chance that your device M40 will be getting core UI only..
