I don't really remember them being any more difficult than S Ranks. It was just a case of "We're doing Dynamis - Beaucedine tonight and I'm throwing you all under the truck to get my Attestation". Yin/Yang was probably the only thing with any real mechanics (had to kill them around the same time, no?), and that's a bleedin' Guildhest in this game, with 5 mobs instead of 2, though I guess they don't hit like a truck.
Much of the "difficulty" I remember in Dynamis was killing things in an order, with the actual "bosses" being trivial if you gave yourself plenty of time after currency farming. I guess if I count idiots in Dynamis - Windurst (for example) as difficulty then that would make Dynamis harder, and I guess I should since idiots are the only reason Titan was ever hard... Still, fights with groups in XI were just a matter of auto-attacking and hitting a macro when you got TP, or /healing when you ran out of MP (oh god I'd forgotten about that until now...), really cannot find it in myself to call it harder when I specifically remember doing Dynamises at 2am, dozing off several times, and still clearing it. I simply cannot do that in XIV...
Main difference being, XIV only does it with Coil (and that's something they even want to rectify). Loot lockouts in Crystal Tower I guess, but you can still do the event. By the time ToAU came around XI had enough content for it not to matter as much, but I still remember spending a lot of time not really doing anything in the game because I couldn't (because of lockouts). Really isn't the same case with XIV.
Are you talking about Relics/Mythics specifically, or the game in general? Because a lot of content could be lowmaned, or even soloed, other than Salvage before Abyssea came in and ruined the game. I mean, RDM/NIN was a thing.
Arch Dynamis Lord was after Embrava was introduced, no? Everything was an easy feat after that.