The short answer: every matchup. After playing traditional jeskai and switching to Nahiri, it`s hard to find situations where i wish i did not have nahiri in the deck (well, maby if i draw emrakul on my starting hand for the 10th time in a row :D)

as for tron, Nahiri is actually the only way you win game 1 and also a common win condition post board.
Post board I either win through Geist and burn or through Spreading Seas / Crumble and Nahiri because even if you crumble them, you still have to kill them before they just start hardcasting their stuff which still kills you.

As you already pointed out the Jund and any other midrange matchup improves a ton, playing vs Jund feels amazing for me (then again out local two Jund players are not the most expirienced)

Nahiri is also very good in most if not all agressive matchups because she provides removal and forces your oponent to hit her instead of you in the worst case.
She also closes the game fast after you stabilize ensuring you don`t die to topdecks.

The last advantage is the "twin effekt" where you can steal games that you otherwise had no right to win.
This happens less than with Twin obviously but I´ve had countless games vs Control and Midrange decks where i was behind on cards or was staring down a Liliana but was able to force through a Nahiri and chump / removal untill i can ult her.

When it seems like I am just mentioning good things about this version of the deck then thats because it is, in my opinion and from the statistics, a lot better than the traditional Jeskai version.
(according to a mtg salvation mod who i assume has access to the data from somewhere traditional Jeskai occupies 0.1% of the current modern meta while Jeskai Nahiri occupies 6+%)

All of that being said, Jeskai Harbinger has quite the different playstyle to traditional Jeskai Control, you are taking the tempo role way more often in matchups that you do with old school Jeskai and if that is not for you then it is completly fine to stick with what you like.
Jeskai Control is as good as it has allways been, the presence of Nahiri has little effect on that, possibly making your oponents sideboard poorly assuming you play her, but the Nahiri version just happens to be incredibly strong in the modern meta.

I personally feel that Nahiri is just too slow in the matchups where you have to close out quickly and that I felt better with having more interaction in these slots.

If you are still testing with her i´d recommend you try using her -2 more.
When I started i was plusing her almost allways but nowadays i see way more spots to - her, that way getting the interaction you had to cut to fit her in and also getting a guaranteed 2 for 1.