SAM is easy to pick up and do well with, but has a lot of depth once you spend time with it. However, it's punished the least by game mechanics; positionals, while important, don't hurt SAM dps nearly as much as they do DRG or NIN or MNK. In that sense, SAM is significantly easier just by of not suffering as much as the other 3 melee jobs do when positionals come into play. Missing a positional on SAM cuts into your Kenki generation, which in turn cuts into your Shiten/Kaiten usage, but that pales in comparison to the penalty a MNK or DRG suffers, where they can lose over a third of the combo's total potency due to being unable to properly hit positionals.

Now, to be fair, DRG, MNK and NIN bring oodles more utility than a SAM does (SAM brings exactly zero unique utility to a group), so while the raw numbers may be lower, a good DRG or NIN still bring solid boosts to the party, and a good MNK can compete with a good SAM while also providing some neat utility as well. Big numbers are nice, but they don't tell the whole story.

Of the caster dps, SMN is probably the most complex in terms of how much you have to plan your skill use around the limitations of the fight. A lot of your damage is tied to Bahamut, so using him at the wrong time, or dying at the wrong time, will really bork your damage. SMN therefore requires a lot of forethought, moreso than BLM or RDM do. BLM also requires forethought, but a mistake, or even a death, isn't nearly as costly as it is for SMN. RDM tends to be the "easiest" in terms of putting out damage, but lacks the MP recovery that SMN and BLM has, and due to it's higher utility via Vercure/Verraise, has an added expectation to provide utility support should the need arise (whereas SMN is really only expected to use Raise when swiftcast is up, and a BLM's utility comes in the form of being a mana battery/Osteoperosis/Addle provider, which RDM and SMN can still technically do).

BRD is probably "better" overall than MCH in terms of all around support, but MCH brings more damage. However, it's quite hard to get those damage numbers up. I have a lot of respect for the high-end MCH's who know their class. It's a joy to watch videos of a good MCH do their thing, but it's something I could never become proficient at. I just can't wrap my head around what needs to be done and when, not to mention the manual dexterity needed.

As an FYI, I say all this based on personal observations. My 70 dps is a SAM, and while I'm not a bad player (I think I'm a pretty great tank) I rarely played any dps class in an end-game scenario prior to Stormblood. SAM was something that came pretty naturally to me, and with just a little research and an hour or so of banging away on a dummy, I manage to get pretty good with the class. My first time in Omega 1.0s I managed to put out some impressive numbers with ilvl 316 gear, and that was with a few clear mistakes made here and there. I imagine if I had been a MNK or a DRG my overall damage would have been lower by a good chunk.