Rubbish advice on a general basis taking into account UAs route system where CPUs are allowed.

Yes, there are many 1ks on this forum who say they have low CPU rates, even when traveling on higher fares and I don't doubt it. If you are flying at peak times on business-heavy routes, or out of certain cities that have high numbers of elites or that have a lot of companies that will pay for premium cabin, of course. If you're only flying hub-hub at peak times, yes, upgrades are going to be tough. Flying with all of the consultants on Monday morning and Thursday afternoon/evening? Yup, upgrade rates will probably be low. Silver or 1k. And yes, with discount F fares being more discounted, there are more people buying it.

That said, there are many routes where CPUs are common. Not ORD-SFO on Monday morning, for sure.

Last year, as a plat, my upgrade rate was 63%. Yes, most of my upgrades were on shorter flights. I missed some longer flights, and hit a few of them. But I also don't have the expectation that UA will or should give upgrades willy-nilly to elites, especially at the expense of paid business.

I expect, and hope, they do try to sell them for $ first, and give them out if they don't think they will or it's clear when they won't. That's the difference between my perspective and many on this board. I would be fine if I never got an upgrade because UA sold the seats. I'm happy to get them, but don't expect upgrades should happen every time. Many have expectations that because they are loyal, UA should upgrade them for free, every time. I'd ask them what other businesses they are loyal to that gives them free items on the level of upgrades, everytime. Or for those that have their own companies, if they'd consider giving their best customers freebies like this everytime. I'd be interested to hear the answers.

Back to the OPs original question, fare class does matter. After status, fare class is the next priority taken into consideration. Though the exact impact is going to change by some of the same factors that influence other chances of being upgraded. Is it a primarily business route or leisure? (leisure routes are likely going to have more folks in lower fare buckets, so fare class may be less relevant, whereas on business routes, there are likely more folks in higher classes so a lower class is going to hurt you more). Are you flying peak times? What is the elite population to these cities? What aircraft (F to Y ratio)? It's not a simple answer - it's going to vary based on several factors.