I don't have experience in all classes, so cannot speak about all dps choices from my own experience.
Each class has its own fights where they have their own difficulties due to boss mechanics. At the end of the day it is all about knowing the fight so you know when and where to or not to use particular abilities.

As I have only Monk and Summoner from dps classes at 50, I will speak mainly about those two.

Regarding diffuclty of Summoner.
It was a diffcult road at the beginning especially on gamepad for micromanaging pet+using all skills, but after few months of playing I would say it is fairly easy class to play once you get your brain used to particular bars, situations and pets idiotic behaviour
What is fun, but at the same time might be difficult at the beginning, is the fact that other than opening rotation you do not have set skill order. Keeping dots up, managing your aether stacks and CDs is priority for optimizing DPS.
Class is very capable of dealing damage even in move.
The main area I find problematic and believe I have most room for improvement is burning adds for dps checks. In my opinion that's the weakest spot of summoner as you need few GCD to build your aoe damage.

Regarding Monk.
Some encounters are a nightmare for monk due to forced stops in fight which can result in loosing GL buff. I am not feeling as comfortable on monk as on summoner yet as I hit 50 few weeks ago, so perhaps I will not find it as problematic in future as I do now,. Nonetheless managing your GL is a priority, hence proper usage of PB seems vital for problematic encounters.
Monk seems to me to have pretty set rotation, although positionals+GLmanagement+keeping up dots and buffs seems to be more problematic for me than summoner to keep the dps high.

Since announcement of BLM increase in single target potency I have noticed some sort of trend that suddenly BLM started to be considered as a difficult class. Now, I don't have this class at 50 so don't want to throw empty words without any background, but from what I saw in game and read, BLMs issue is being unable to deal damage while moving. Well it is the feature of a class and some fights might be hell unless you know the fight well. Although, it is a case for all classes that in order to utilize your abilities, you need to know the fight, so I wouldn't make BLMs martyrs
Have quite set skill rotation, rocks in AOE burning and is quite fine now I believe in single target.

Dragon no clue from my side, Bard seems fairly simple with its mobility.

Hope this helps a bit