Skippable ads played in beginning & in middle of videos both earn me a lot.
The CPM for non-skippables is about 66% of the CPM for skippables.
Skippables seem to be watched a lot more. In fact, skippable ads have earned me FIFTY-ONE times more CASH than the non-skippables over the lifetime of my channel.
To put this in perspective, if non-skippables earned me $100, skippables have earned me $5,100. Those are made up numbers just for you to use as a reference.
Monetized playbacks on skippables compared to non skippables is about 34:1, meaning if I get 1,000 views on non skippable ads, I get 34,000 views on skippable ads (again, made up #s).
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Now, I don't know how often YouTube shows my viewers skippables compared to non skippables, but it seems that non skippables are the way to go. Even though they are skipped often, they're making a LOT more money than non skippables which probably means people are closing the video before their ad-view is counted & paid for.
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It's the classic movie ticket math problem.
Do you want a ton of customers (ad views) that pay a lower price (aka skippable ads), or do you want a smaller group of customers that pay more (aka non-skippable ads).
For me, I'd rather have a lower CPM with skippable ads because it equates to more views, which means more subscribers, higher rankings in search results & it expands my channel which in turn will bring more money. The higher CPM with non skippables is great, and you make more money, BUT you get less views which means less brand recognition, less subscribers, less return-viewers (aka fans that will watch pretty much anything) and it's just not a good strategy for the long run, in my opinion.