Upgrade certificates in particular are far, far, far easier to clear on Delta than they are on United. You can roll back the clock about 20 years on finding upgrade space on UA to get to the current state of affairs with DL. I can't promise everyone will find them useful - if you only travel on Delta monopoly routes at peak times and regularly pay for expensive fares (as seems disproportionately common for DL elites) then you're not as likely to see anything. But specifically compared to the equivalent benefits on other carriers, I think DL is by far the easiest to be upgraded.

I also can't say I have noticed anything that far off about complimentary upgrades. It's fairly easy to clear one that I plan for, but of course most domestic flights you have to pay for F. As when I was UA 1K, I clear nearly all the complimentary upgrades that I attempt, but only try it maybe 15% of the time. It's the same everywhere; the majors are actually trying to price domestic F competitively rather than sell four seats for thousands of dollars and hand the rest to elites.