@antinn Thanks, Neil.
@gano - the answer is that proportion of older copies that have been contaminated. Visually, you may not see this. I was buying a considerable number of "rare" records and in some cases, particular pressings, that were either not often seen for sale (even before the Great Disappearance™) or quickly went up to astronomical pricing. Therefore, I tried like the devil to get them to a high playing state. (Though in a few cases, I did wind up with multiple copies to get one that was a good, quiet player).
I’d say there is a direct correlation between how many old pressings you buy and how often you have to resort to more rigorous manual cleaning. In some cases, I bought records that were never played, just bounced around among dealers, some of whom didn’t know what they had. Those days are over and I’ve slowed down considerably on buying, given grade and price inflation. Some are simply not on the market given the limited number of original pressings that were made, private label or obscure things that never succeeded in the market place at the time of release. There is no good answer to your question. Every record here gets cleaned before it gets played, including new records which for me, is a small fraction of what I have. But I vary how I approach each and also use a Furutech DF-2 flattener-- once you are sensitive to warped records, you’ll see how many are; I won’t flatten something that tracks properly but if the record is out of round, there’s not much you can do. Perhaps digitize it and rely on that-- I know that goes against the grain of the analog purists (of which I was once one), but I don’t want to risk my cantilever, especially on a linear arm where you can see the arm "hunting" on an out of round copy.
PS: FWIW, if I had to choose between a vacuum machine and an ultrasonic, I'd opt for my Monks Omni, which is not a casual purchase. But, thankfully, I don't have to make that choice; I use both methods and they are synergistic in a real sense.