My end game raiding in wow ended in wotlk and started/ended again in legion. I'd say wow raiding is harder at the top end than FFXIV Savage but FFXIV raids being only 8 people make the personal responsibility much higher.
No one can fuck up
Also I disagree with Rezz, 2/4 on Savage ATM as a Warrior and you would get the boot from most groups if you stayed in tank stance during raids.
Non Tank Buster damage is very manageable and abilities come in at a timed intervals.
Midgarsomir has the hardest hitting Tank Buster so far and it has a 4 second cast time plenty of time to pop into tank stance and hit a cooldowns to negate.
No reason to stay in the stance if you know the encounter.
Raids and general play are very different in terms of tank stance requirements. Yes for savage you definitely will want to optimize your DPS stance time, as in, it should be 100% unless you use tank stance as a "cooldown" for certain things, and maybe on pull for initial threat if you don't have a ninja.
For casual play, DPS stance can be definitely worse. This is especially true in 4man dungeons you don't overgear(like current experts) on trash pulls. Fact is, pulling 2 packs of trash in a relatively close ilvl dungeon will generally mean you're taking more damage than any savage boss does, for an extended duration. Sure there's no tankbuster or mechanics to cooldown through, but the damage output of 2packs of trash mobs is consistently several times higher than what a savage boss autoattack does.
That's why you have to cooldown trash packs and that's where DPS stancing does the least amount of good. If your healer is well geared and good, they can probably weave in some damage between heals, but the less defenses you have, the more they have to spam actual GCD heals, and that in turns can be less damage than if they could cast aoes instead(especially WHM/AST).
Now on top of that you have the issue of bad DPS in pub dungeons, so a trash pull that should take about 35secs, most of which will be during defensive cooldowns, letting you offset the DPS stance defense loss, if the DPS are shit, and they're often shit, that same trash pull will take over a minute, and that means you'll spend a noticeable amount of time not having any cooldowns up.
Threat shouldn't be an issue, but also can be one if your tank is undergeared(weapon especially) and you're grouping with a max ilvl char. This happens if you do expert roulette at min ilvl, or worse, 70 roulette at min ilvl(so you can be in i290 job quest gear doing Ala Mhigo and grouped with an i400 Samurai who's also being a retard and not popping Diversion, then good fucking luck tanking in DPS stance, even tank stance will struggle).
What I recommend for any new tanks, is to first tank stance everything, then DPS stance bosses only, and only DPS stance trash pulls if you're well aware of the trash damage output(some hit harder than others) as well as your healer AND DPS ability to deal with the packs. Generally in 90% of the pubs I'm in, this means tank stance trash if I don't want to waste more time(possibly wiping too because healer is shit).
Trials and shit depends on your experience and gear, and how good and comfortable your healers are with the fight. I tend to still tank stance most EX primals because even with tank stance it's not uncommon for me to have to clemency myself to actually survive due to healers focusing on other shit(Suzaku is a good example with the dance and shit, a lot of healers tend to "forget" about the tank). If the group is good though I'll DPS stance, to an extent, my gear being kinda shit I'll still adjust depending on which part of the fight it is and what cooldowns I have.
Once you get to high endgame, then you will want to learn how to maximize your role efficiency like everyone else, and that does involve DPS stance tanking and learning the encounters so you cooldown appropriately. But this isn't something that requires extended practice since for the most part the tanks play the same in DPS stance than they do in tank stance(WAR is a bit different but still) and as such it's not something that you have to bother learning doing leveling dungeons or whatever. Hell as I pointed out, 4man dungeons are very different in terms of damage intake than savage raids. The danger in terms of tank damage in 4man dungeons are the trash packs, not the bosses