Will Google UAC always perform worse / deliver less on iOS versus Android?

I've only had the exact opposite experience. Spent 100k testing different strategies on Android and never go it to any reasonable results (e.g. Google spending 1k / day when delivering ONE conversion with my 70€ CAC target). iOS worked quite a bit better Android, though that was generally the case for the app I was promoting (Health & Fitness vertical).

Short answer: Not always.

Longer answer: UAC is largely built for e-comm and gaming companies, so if you're not in those verticals then you can get some very funky results from the system. Price point can also factor in as iOS tends to be more accepting of higher prices where as a large portion of Android traffic is not.

iOS actually has more scale/delivery and outperforms our Android campaigns and has for over a year, but we're not in either of those verticals listed above and we do have a high price point compared to most mobile apps. In theory, Android campaigns should optimize faster regardless of what type of app you're trying to promote as they have access to more data via Android OS, but even that doesn't appear to be the case for our iOS campaigns as those optimize faster than our Android ones. My guess there is that Google just doesn't know how to interpret all that data when looking at it through their gaming/e-comm glasses and will often sort it wrong or pick up wrong signals. In that case, having less data and just going after what 'works' (your optimization event) it helps them react faster rather than try to 'predict'

In my experience, delivery (volume) is indeed a lot higher on Android, which can be explained by the additional inventory (Play Store Search, Play Store Display, and to some extent google.com search) but also due to the simple fact Android much higher marketshare in many geographies.

As of performance, I'd say "it depends" (which would be a "no" to the question then).
UAC on iOS lacks the premium Play Store inventory to compensate for the poor performance of GDN/admob traffic, but for most developers, conversions & ARPU are much higher on iOS, which helps the machine learning process (more monetization events completion), and more often than not can deliver higher ROAS despite higher CPI. In one particular instance, our Android campaigns did have decent volume but never reached CPA & ROAS goals and had to be shut down, while we could still run iOS profitably, albeit at a much lower spend level.

If we were able to select or exclude networks/placement, the story would likely be entirely different.