Preface: I never campaigned for RDM (or SAM) nerfs (though I'm raising my eyebrows at NIN).
The reason why RDM is "easy" is twofold; first, you just can't mess RDM up.
What even is messing RDM up? Getting 100/100 mana? I mean, you still do your melee combo+finisher. You just lose the guaranteed proc (which will probably set your next melee combo back by one gcd). The same is true if the mana is correctly misaligned and you use the wrong finisher (you're at 81/83 and Verflare instead of Verholy). That's like so minimal. One gcd of slightly less damage. You're not even guaranteed to lose a finisher over the entire fight off this. And you still have 20% change that your mistake will be inconsequential anyway.
Second, compounding that is the fact your ogcds are best used on cooldown except for Manafication (which just has you look at a number whenever and then use it when you get 80/80+), you don't even really need to plan ogcd usage in a fight with RDM.
Things are up? Use them. This is for every fight and every mechanic. I just "wing it" with RDM and do fine. The job inherently does not require planning, especially since most cds are best used as soon as they come off cd (except Embolden in a static, you'll ant to align that with the rest, but this is true for any utility tool any job has, so).
This is not true for BLM at all. I need to think about where I'll use which cds. I need Ley Lines down for the Levin Bolts so I can Between the Lines back and forth to the correct side of the arena for Susie, I need Triple up for the cloud+knockback in case I get targeted, I need Swiftcast up for the puddles during the first gaols. In Lak EX, on p2, I need to start with the 3.X rotation because she'll open with Divine Doubt and I won't have time to do the 4.0 one, but then I need to execute a 4.0 after a second 3.0 one so I buy time for the Foul clock. It can't be right away, because I need to refresh my Vril.
There's just layers upon layers of cd usage and rotation planning I have to do, or I'll have a bad time. RDM doesn't have to deal with none if this stuff.
I don't think there's an issue with it, either. Sometimes I'm tired, I'm sick or just had a bad day and pick RDM up. It's plug and play, I don't think much, I do lots of damage, Corps-a-Corpse and Displacement (and Riposte) have some of the coolest looking animations, and I just have a good time all around.
But, objectively speaking, in virtue of the job's design, it is a simple job. Whatever "complexity" I need to deal with RDM I also have to deal with other jobs, but those tend to have layers of planning and decision making tacked onto them if I want to execute them perfectly.
And it's fine, really x)
But it's also unquestionably easier, as far as planning/decision-making goes.