Spoilers for savage mechanics.
Wanted to come on the forums to share opinions on the tier after my group finally killed M4S yesterday.
First, I'd like to say that this tier was miles ahead in terms of fun compared to any of the Endwalker tiers. Like, across the board. For the most part, the fights all feel very active and involved, and much less of the *stand here and wait for debuff to resolve* type mechanics from Anabaseios/Abyssos/Asphodelos. Probably the most engaging mechanics I can think of from EW were the fire tornados in P3S and Dog 2 in P8S.
M1S - while the knock-up/slam-down mechanics aren't truly random, they still feel like they need a good amount of adjustment on any given pull. There's a bit of memory required when the boss slides around the room doing it's cone/paw swipe mechanics, and sorting out the cleaves during the second knock-up section requires a lot of attention from every player. Probably my least favorite of the bunch, but I'm still putting it ahead of the majority of EW raids.
M2S - This fight was just fun. The "use eyes" factor and the sheer amount of adjustment makes every pull feel very different. Mechanics like the Groupbee's line AOEs and the hearts might be tough to get melee uptime around, but it's so hectic that I can't help but smile almost the whole time hearing people call out where they're going to go and what adjustments need to happen in the next 5 seconds. In fact, I don't think I've laughed this much in probably any EW content. I figured the random mechanics like the hearts and bees were just going to be normal mode exclusive and they would become very streamlined for Savage, but I really do appreciate that they left these mechanics alone (more or less).
M3S - This fight has much less of the randomness, but the mechanics are all pretty fast paced compared to other fights. The time to gauge whether the boss is doing quadro/octo mechanics is pretty tight, and then on top of that determining whether it's going to be an AOE jump or a knockback. His doped up combo is pretty cool (and the enrage is perfect, imo on par with TEA's enrage), and you need to decipher pretty quickly which add to get hit by and which quadrant will be safe 3 seconds later. All in all, I felt that this fight was the fastest pace we've had in a raid in a while. M2S feels fast because of the randomness, but M3S actually is pretty fast.
M4S - THANK GOD THIS IS NOT A DOOR BOSS. Endwalker completely burned me out on door bosses so I had a sigh of relief when I saw that this was just one complete fight. Definitely the most difficult of the tier, this one took about 8.5 hours total for my group to clear. I don't know how PF is going to do this lol. This is the only fight of the tier off the top of my head that has body checks and still maintains the fairly fast pace of M3S (in the first phase anyway). The widening/narrowing witch hunt mechanic will be a pretty crazy bait mechanic for PF to make strats for. I was proud of the normal tier for not having a wall boss, but I'll forgive M4S for, again, not being a door boss lol.
The second phase slows down a bit but still requires some fairly quick movement for the sabbath mechanics. One thing I enjoyed that DT has done so far is that some Tankbusters actually involve the party instead of just that tanks (i.e. Mountain Fire from EX1), and M4S has that with the mustard bomb mechanic. Sword Quiver was a pretty hard hitting buildup to enrage, but it wasn't anything too crazy, just a lot of damage going out in quick succession.
I ran Sage for this tier, and IMO the healing was fairly easy in M1S, not much required outside of stack/spread mechs in M2S (the hearts have a pretty long lasting DOT effect as well), a good amount required throughout the entirety of M3S due to the multi-hit raidwides, and a ton of healing/mit planning required for M4S.
Overall, this tier was pretty good, easily better than Anabaseios. I really do hope they continue in this more active direction with mechanic design. Feel free to share your opinions as well!
TLDR; M1S fine, M2S active and engaging, M3S fast paced, M4S fast paced and difficult.