From what I've gathered playing roughly the four jobs :

Monk is the easy to learn/easy to master melee, limited in its gameplay, that kinda rotates and repeats itself, but it does decent damage and has some nice utility to bring to the party.
Brotherhood is really good to boost physical damage, and boost your own damage as well by giving you a lot of chakra proc, and Mantra is a 20% group healing buff. The job works fine but the over-reliance on positionnals can make it clunky sometimes, and downtime is your worst enemy as you can take forever to build back your ressources if you're not carefull.

Ninja is on the opposite the hard to learn/hard to master job. Lowest raw damage of all melee but has a 5% damage increase every minute for your whole group.
If monk has quick GCDs but not so much oGCDs, ninja kinda has the opposite with a lot of buttons to press to make the ninja do... ninja things. The mudra systems requires you to input a quick series of spells in a specific order to trigger a ninjutsu. Some can be a speed buff, a raw damage spell or one that lets you use your best ability : Trick Attack (the damage buff). Ninja kinda looks like it's easy to mess up, but also easy to get back on track once you know what you're doing. You gotta love pressing a lot of buttons tho.

Sam is... Special. It's easy to learn, but hard to master in high level content. Sam is often disregarded as a braindead job that's not really good, because unlike every other melee, it doesn't have any party buff. So it only depends on its player's skill.
So the better you are, the better your Sam will be. It's a huge "raw damage" job, with a very strict rotation due to the lvl 76 spell. A good Sam can crush every other damage dealer in the game, but it requires an exceptionnal level of play in order to do so, as missing even one GCD will mess your rotation. Sam is the definition of "learn to optimize everything". If you like fight optimization then this job's for you. Or just if you like big 100k in one hit. For casual play, just try to refresh your dot every minute and the rest should go pretty smoothly, as the job is really easy to play if you don't try to optimize everything.

As for Dragoon... Not easy nor hard to play. The job's main rotation is very simple and once you know it it's nearly impossible to drift away, the real difficulty of the job comes to the oGCDs that come with it. If Sam is a job about timing perfectly your GCDs, then Dragoon on the other hand is all about keeping those oGCDs on cooldown, which can make it tricky sometimes. Still, it has probably the best mobility tool for a melee : a backdash that doesn't need a target. Dragoon's dealing decent damage, has a very fluid rotation once you know how to play it and has two really good party damage buff that makes him one of the most popular melee job of this game. If you like spear, jumps, mobility and buffing your party then Dragoon's the job for you.

hope that helped