I don't level the Undaunted skill line, so none of my characters have the passive.

I don't level the Undaunted skill line, so I don't use it.

I do wear a mix of armour types on my characters while levelling them so they have all the armour skill lines levelled to give me options, then once they get to level 50 I pick 1 or 2 (usually 2) to focus on.

I mean, technically. Most of my characters don´t meet the requirements. Those that do have the skill - it´s a passive, no downside.

As for armor distribution - one of the best changes ZOS ever did was the armor change so that all armor types are strong, on all characters. At the moment, it´s probably mathematically best, at least for me, to have at least one piece of all armor types, with light armor on the smallest piece - meaning waist. I play mostly solo, so even though I usually go stamina, I can always use more penetration.

Usually I don´t think too much of it though. Most of the time I am limited from the actual pieces I have. And the difference is pretty small.

Do I have the passive on my characters? Sure, why not. It's at least 2% free stuff, and more likely 4-6% free stuff.

Do I use armour weights specifically to get the higher tiers of it? Not anymore since CP2.0 gave everyone lots of stats from the start. Before that, the passive was very important; nowadays, it's just nice to have.

Its free stats, even if you only wear one armor type. Changes to the armor system made mixing in pieces more likely.
It just seems less important as stacking max stats isn't as good as stacking damage and recovery to support it. Memba Hulking Draugr?

As pthers said, it's free stats. Every character I'm going to do content with, I will level rhe passives. I won't use all three armor weights tho. Heavy armor is always a damage loss on damage dealers, so most of the time my characters are on 6 medium and 1 light, no matter the max resource they spec into. Only times I use more light is when our group has no roar of alkosh.

Most because some don't yet have undaunted fully levelled.

I think it used to be mandatory before the devs increased base health; prior to that you'd be running 15k-16k health as a primary attribute (mag/stam) dd and with it a more respectable 17k -ish health, for example. Interestingly, some 1-shots from then like bc1 final boss unblocked are pretty much auto-survivable today with the increased health.

For tanks I've always preferred 5:1:1. For healers, same but tending to go 5:2 (L:M) more. For damage 7 (so not using the passive at all) or 6:1 if I want more survivability. This is for pve.

For pvp always will have at least one of each armour type to use the undaunted mettle passive; it's free resources and too useful to ignore. Much less rigid in types and number of each type of armour used, though.

Not sure why you wouldn't if you have a skill point to use - even 1% is something for free.

I dont go out of my way to level or max Undaunted on every toon (even ones just used for transmute gem runs) but if they reach the levels for the passive, I make sure it's activated for em with 1 of each of the other armour types.

The trouble is, in fact, finding where you can spare the room for a wrong-weight piece of armour, assuming you want to keep complete sets as far as possible, because you may lose a Set Bonus.

For instance, if you go with the classic configuration of 5-5-2:

- If both of your sets are Dungeon or Trial sets, or indeed Overland sets (other than Sithis, Bahraha's, Syvarra's and Trainee, all of which are available in all weights), and you're using a monster head and shoulders, the Dungeon/Trial sets all have to be in the same weight, so your different-weight items have to be the monster head and shoulders. For instance you might wear two Light sets, plus a Heavy head and Medium shoulders, or two Medium sets plus a Heavy head and Light shoulders, or two Heavy sets plus a Medium head and Light shoulders.

- If one of your sets is a Crafted set, those are available in all weights, so you can simply craft a couple of them in the wrong weight. For instance, Hunding's is a good Stamina set so you'll want it in mostly Medium, however you might make the Chest piece a Heavy one (for the greatest gain in Armor value) and the Belt piece a Light one (for the smallest loss in Armor value). Or Julianos is a good Magicka set so you mostly want it Light, but you might make a heavy Chest piece (greatest gain in armor value) and a medium Legs (also a good gain in Armor value for medium over light.) If you're a Tank using a crafted set (e.g. something like Torugs or Brass Fortress), you might go for a mostly Heavy set, but with medium Boots and light Belt.

If you're going 5-5-1-1 including a Mythic: The mythic is most likely jewelry (most of them are), so it may well be that the best you can do is a wrong-weight Monster Head, and two dungeon/trial/overland sets for 5/5 - unless you're using a crafted set, in which case craft one of the pieces in the other wrong-weight. Using a wrong-weight mythic armour piece (Death Dealers Fete, Spaulder of Ruin, and so on) is probably not a good idea since it probably won't suit your playstyle.

more stats is more betterer

I use the Undaunted passives on all of my characters (18). And sometimes the skills too.

The Undaunted skill line gets leveled naturally by doing what I normally do. And the passives give max stats and resources from synergies. So there's no reason for me not to use it.

But I usually don't care what the passive is for. If a passive gives a benefit then I'm using it.