Why is Nowi in the Tier 1 ?

I changed my 5* Nowi +SPD/-RES for a +SPD/-HP one because the -4 in RES felt way too big a problem. She isn't supposed to even try dealing with greens, regardless of build, which is why people recommend Triangle Adept 3/Quick Riposte 3 as it helps her almost completely destroy the entire cast of reds. Little tip about this... if you can have anyone with even a pseudo healing skill like reciprocal aid/breath of life/ardent sacrifice then Quick Riposte has so much more use. To make the best use out of her you want to run someone that can check greens (my green/colorless hard wall Sophia or Ike come to mind), and build her to wall off any and all reds. The Fury Vantage set makes her pretty scary with 46 HP/50 ATK/34 SPD/34 DEF/30 RES at +2 but much more vulnerable to Falchioners, so in these cases you'd need to run someone that can soften Falchioners so she can one shot them, as well as do their own jobs like my Bride Cordelia with 55 ATK/42 SPD w/ Firesweep Bow (+).

If Nowi has the speed to deny you doubles, or, better yet, to double others, she becomes terrifying because of her amazing bulk. I think she's a great blue unit but requires some support to truly shine.

I can also concur that -HP nowi is just better. She's a monster of a unit, tbh. I think you're just expecting the wrong things out of her. Green mages mangle most things that aren't red (and plenty that are). Nowi's Resistance is actually super good for a unit that comes with a DC weapon. I don't think the tier list is correct on everything, but Nowi is definitely tier 1. She's good on offense and defense, and doesn't require anything from her team mates to be viable.

To be honest, I think it's because her rating hasn't been reconsidered here in a while. In tier lists on other sites, she's as low as A+ tier at this point. Months ago, she was a legit S/S+ tier unit. A lot of new units just blow her up at this point and Dragon Emblem is no longer really as viable as it once was.

That said, she does have a high BST for Arena. And she is still good. I'd invest in her, but don't expect her to be Ryoma/Hector/Effie/Reinhardt tier. Also, keep in mind tier lists kind of don't matter as much as some would think. The most annoying team I faced in Arena last week involved Adult Tiki. The one with Ryoma, Hector, Rein, and Linde I dumpstered in 3 turns.

P.S. Don't worry too much about Falchion users. The only one you're likely to meet in Arena is Lucina. Just make sure something else on your team can deal with her or put Swordbreaker on Nowi.