Being both good AND bad at the game?

[Nethereal]

Recently I achieved the goal of clearing every one of the Coils raids, T1 through T13, at Minimum I-level and no echo. For those who have done the Coils raids MINE, you know how hard this is to achieve and is not an easy feat.

I have also done most of the 50 ex' primals MINE, and all of the 60 and 70 EX primals MINE level. (yes, even Thordan and Shinryu) Am currently working on the 80 EX ones MINE.

The content you are talking is so far back you had cross-class skills when it originally came out, all of the above is irrelevant content and is not a good place to get a scope of your skill as a player.
The content has been nerfed and jobs have been adjusted over the expansions and that does not get unerfed/unchanged regardless of if you do minimum ilevel and no echo the experience you will get now versus the experience people who originally did it when it came out will be vastly different.

And yet, if I do many of the dungeon roulettes, I tend to totally, totally suck in them for some reason. Seriously, I screw up the boss fights and tend to die lots. My friends usually bet on if and when I will die in a dungeon instance.

If you have done any coils content, especially MINE you know you cannot really be carried, especially in stuff like Nael or Bahamut. You need to know your stuff, know mechanics, and know your role decently enough to get the clear. You don't and people die and the party wipes. So I am not an idiot at this. I went though every fight, got the clear legitimately, and in many of the fights did not even die at all on the clear run.

Once again the content you've said you've cleared is not a good test of player skill nor knowledge the content you need to be doing to talk like this is current savage and ultimate. Even current extremes are not that hard and are usually memed both unintentionally and intentionally.

It doesn't matter how well you do in a perfect run but how well you do in a bad run.
A perfect run has no room for improvement but as humans we will always make mistakes eventually and how well you can adjust to these mistakes is part of what defines you as a good player.
You talking about how well you can do something you've spent numerous hours rehearsing is not the end all be all of a good player, in fact it's the bare minimum required to be decent at this game.

Yet if you are with me in a dungeon I am sometimes soooo bad at it that you would call me a noob who has no idea how to play the game and needs to 'git good'.

It's really so puzzling at times. I am really kind of curious to know if anyone else is like that. Good at hard stuff, bad at 'easy' stuff for some reason.

It's because you've only improved your ability to mimic the same actions over and over again you haven't improved your ability to deal with erratic situations,(healer's adjust) if you had you wouldn't struggle so consistently with (any)dungeon content at all as it's all much more easy then all the content you've described having done.

My advice start with understanding the concept of damage.
Look into maining one class and start practicing savage and when you feel you've reached a peak try ultimates.