Not necessarily true if both ad blockers are using the same filter lists. I find Adblock Plus and uBlock Origin (I prefer the latter) to be relatively on par in terms of things they block and things they tend to miss, even cross browser. I think the initial pageload would be the biggest differnce, as it relies on the performance of the adblocker in applying its filters to the DOM once it's loaded. The underlying extension framework probably affects this a lot too.

I'd think the user agent string is more likely to cause a huge discrepancy, as many frameworks will be utilizing polyfills and employing different technologies depending on what each browser supports. (Or, in the case of the user agent string, what the framework thinks the browser supports, regardless of how correct its assumptions are.)