I HATED this arc so much in seeing how everyone(like Spike) got brainwashed and how it was Starlight that save the day.
Luna and Twilight going into Discord’s mind only to fail was the problem.

@Background Pony #F6EF  
He hasn’t assimilated her because he knows he would be consumed by her

What happens to pinke pie

Glorious convergence. One mind, one will, one soul

@Background Pony #761F
 
I don’t think that Discord had much choice in the matter. The arc showed how he was imprisoned in his own mind by his new persona Accord. He just couldn’t know that Accord would be so power-hungry and evil.  
And regarding overpowered, far as I’m concerned, that’s a very good thing. There is hardly anything more exciting or thrilling than a villain that outsmarts all the heroes with ease, has a natural resistance against their most powerful weapons and can in the end only get defeated by either sheer coincidence/luck* or by getting talked out of his evil plans*, like it happened here with Accord.
 
 
*Interesting to note here, these two types of defeats are exactly the two types Starlight Glimmer got defeated with. Her first defeat happened through sheer coincidence, literally one single drop of water that landed on the right spot at the right time was what got her down. If it wouldn’t have been for that waterdrop, Twilight and her friends would have given in to the brainwashing eventually.  
Her second defeat could only happen because Twilight tried to get her back to her senses via talking, as even seeing the horrific future she caused did not prove enough to make Starlight stop with her revenge plan. If Twilight wouldn’t have gotten through to her, Equestria would be forever annihilated now.  
This makes Starlight and Accord the two most thrilling villains My Little Pony: Friendship is Magic has to date.

I disliked this issue so much because of how overpowered accord was and along with the creepy smiles he put on everyponies face. Discord should have thought that through instead, but I do like the artist and the moral behind it.

@garfan  
True, but Accord welcomed the chance to have the Elements used on him, and look what happened. Where Celestia and the rest of the Mane Six had avoided being brainwashed, the Elements did Accord’s job for him in getting them to succumb to his way of thinking without lifting a finger.
 
Thankfully, it was short-lived for Celestia, the Mane Six, Spike, Mayor Mare, and the others once Starlight convinced Accord of his wrongdoing and got him to turn back into Discord and restore everyone to normal, but it still does not make up for those few seconds Celestia went from her loving, hard-to-anger, self to a mindless, obedient puppet of Accord before being returned to normal and being the first to welcome Discord back, showing no hard feelings for being under Accord’s control for a short while.

@SuperSonic15
 
actually in a well written comic they would have worked just fine. Harmony encourages people to get along, not be exactly alike

They never should have used the Elements on Accord. Now it’s up to Starlight to pull off the comic version of To Where and Back Again and convince Accord to become Discord again, thereby returning everyone to normal and returning life in Equestria back to as normal as it can be with a being of chaos.

Starlight glimmer is the last pony in equestria