To be fair, I'm not an authority on this kind of thing but just some bloke sitting in a flat hundreds of miles from the main events. I can quite easily be wrong.
Also, I do think that the Russians probably have sufficient forces and firepower to prevail in the relatively short term, albeit probably after a much longer, harder fight and with far greater casualties than the political/military elite expected. However, that doesn't mean they eventually win, just that they'll occupy most of Ukraine at some point and install some kind of puppet government in Kyiv.
How they expect a pro-Russian government to retain power there in the longer term (apart, possibly, for some small territories in the east) is currently not remotely clear to me. Surely they can't keep huge numbers of troops stationed there indefinitely to fight an insurgency, and the willingness of the Ukrainians to keep fighting is plain for all to see.
Of course on stuff like this, I defer to people with a much greater knowledge of these types of military issues than I do. Mine is very much an amateur, layman's view.