Although Knock Off is indeed a fundamental part of UU, and by know means isn't strong in both its power (well at least on the first hit) and its utility, and tends to be a tier staple, I don't find it too tier defining.
For example it is amazing to see how many Psychic continue to run around the tier. Common Psychics including but not limited to: Celebi, Mew, Latias, Slowbro, Metagross, Jirachi; all seem to be in relative abundance despite their weakness to Knock Off. Likewise Chandelure is one of the more popular Pokemon despite Knock Off being so apparent. It isn't changing the usage of a lot of Pokemon as much as what one may expect, in fact I find that Knock Off is the balancing move in the tier that prevents Psychics and Ghosts becoming quite a significant amount of the tier. It keeps the metagame balanced and I find is healthy for it.
Another sort of standout is indeed still the amount of Eviolite users in UU this generation. Doublade, Chansey, Porygon2, even Tangela and the occasional Togetic seem to be running around despite the amount of Knock Off users (and on the topic of Chansey I think that Knock Off is the only thing which has kept it from going up to BL this whole time). So even Eviolite users who are greatly hindered by the move seem to be flourishing.
I find this to be rather interesting considering it has been named to be such a "game-changing" move, which I have no doubt that it is, if is a f**king nasty little piece of work, but from what it seems by the amount of Pokemon in the tier, stated above, who would be some of the most hindered by the Knock Off still being able to run around in UU, which leads me to the belief that it is a lot more manageable than what people may thing.
Look at Stealth Rocks for example. Almost all Pokemon with a Stealth Rock weakness but especially 4x weaknesses to rocks tend to not be used as much, and this has been rather apparent since when it first came about. I mean look at a Pokemon like Yanmega who was a rather powerful UU Pokemon last gen and in fact I would have expected it to thrive this gen with all the Psychics, Grass-types and Dark-types running around in UU which it can hit with STAB Bug Buzz and then a lot of the Fighting-types with STAB Air Slash, and boasting 117 SpA as well as 95 Spe and 2 incredible abilities. Yet I have seen hardly anyone running it, and although there be more reasons behind this, Stealth Rocks is definitely a major contributor to this and is/was why a lot of Pokemon end up in lower tiers. Knock Off on the other hand hasn't really seemed to have had this effect on the metagame.
I believe that Knock Off is indeed manageable in that despite its major distribution it is rather predictable when it comes to its users, or more to the point threatening users. On Pokemon such as Weavile, Crawdaunt, Krookodile and even Absol etc. we expect that STAB Knock Off, and the real threat users of it tend to be rather limited. The majority of other users are only really using it for the utility in that it gets rid of the opponents item, and despite that, it doesn't really do much damage unless you have something with a weakness and is rather Physically frail. If there isn't much attack investment, Knock Off tends to still be mediocre, better than last gen no doubt, but still not outstandingly good. And so most of the strong sweeper/tanks/whatever who use it are the ones who really need to be prepared for and can be accounted for. And know doubt Knock Off is something which you just have to prepare for like many other things which you should be considering during team-building. For example you should pretty much in UU always have some good resist or physical tank to take on Darmanitan's Flare Blitz or Victini's V-Create, or quite frankly, they can end up wrecking entire teams when Scarfed.
And finally losing your item isn't really the "end all" that some people seem to think that it is. It by no means is a "YAY" thing unless you got something like fucking Acrobatics, but most Pokemon, even including some of the Eviolite users, can function well without it.
I think I have had enough of a ramble now. Sorry for the long post, I can go on sometimes.
