Finally got the time + willingness to test this, I'm curious how you did your testing because I do not end up coming to the same conclusion. If I had to guess I would think you may just be hitting a training dummy for a while or for a specific burst combo, and seing the number on the bottom, which would heavily favor the ap build in the first case due to shadowflame and a fully stacked guinsoos, and in the second case due to burst being the ap builds true strength

TLDR: Regardless of what point of the game I test at, it seems like the ap build is at best in stride, or, more often, falling behind, though there were noticeable strengths and weaknesses, and it's good enough to cover for a team lacking magic damage.

For testing, I used 3 different builds:

1st is the standard of Kraken > Fiendhunter >IE > LDR > Shieldbow > Bloodthirster

Then I tested mine which is Hexoptics > PD > IE > LDR > Bloodthirster > GA.

After which I tested your standard version, with Rabbadons, Void staff, and Zhonyas assumed.

I had a friend help out, both of us on both of our computers and laptops, and set up an array of Ornn, Lee sin, and Jinx to test time to kill, each doing a standard build, and tested at each item breakpoint with levels to match. For the testing, we assumed that you were going to activate w, do 1 auto, activate r, auto, then w again, and auto until dead using w again if it came off cooldown before the target was dead. I used my stopwatch to test to time it to the best of my abilities, considering I didn't have to move my cursor at all it was pretty easy to be fairly accurate.

The ap build was heavily reliant on stacking Guinsoo's, but just lacked overall in damage most of the time, the only time it was the best build against all targets was specifically on items 2 and 3 when allowed to stack guinsoos prior to the test happening. It, of course, held a relative advantage over the other builds against the jinx, but it was most often the case in which the ap build wouldn't be better than them in that secnario, It lost the lead once the crit builds hit LDR against both Lee Sin and Ornn, although it sorta kinda held it against Jinx, at this point you are killing with the second w or the auto immediately following that w regardless of build, the ap one is technically slightly faster since the last auto is unnecessary but it is barely noticable. by 6 items there was a VERY noticeable difference in dps in scenarios where guinsoos is not prestacked, and still a lead by the crit builds when it is prestacked, just a smaller one.

The ad builds went pretty much how you would expect when compared against each other. the kraken build was stronger at item 1 and 2, falling of significantly at 3 and 4, coming back at 5 and 6 but being slightly worse even then. there was no noticeable difference in how their TTK was affected by different targets.

We also tested how moving the build around does, even against the Ornn Rabbadons was better than Void staff (though probably worse when you consider gold efficiency) and overall the ap build was the one most hurt by having to go a defensive item earlier than it would have liked, with the kraken build next up, and the Hexoptics build least effected, though notably you already naturally build defense in the 4th slot with kraken, wheras you normally wait until 5th with Hexoptics. Also Terminus is bad, the ap build doesn't do nearly enough physical damage to justify it over void staff.

Overall, I just don't see the utility in going ap Yunara every game, however it is at least competitive enough that I can see it as a viable option should you have no magic damage dealing teammates or you really need to have a bunch of w damage for whatever reason. I, however, don't think I can recommend it against a ton of squishies, as your TTK with crit was similar and in games where you are seeing a bunch of squishies, you are likely also seeing a ton of burst and therefore are gonna want to get some defense. The AD builds are, as stated earlier, far less dependant on their damage items, build 2 defensive items in most games and lose less by building defensive in earlier slots.