Main problem with the executioner is scaling, like many essentials classes despite assassin's strike looking impressive dice wise it starts to fall behind at paragon/epic. It does work a lot better than the OAssassin, but I've used the analogy before but that's like saying a heavyweight boxer could take a 5 year old (so isn't very relevant). I did have an executioner in my Dark Sun game, but an Anakore killed him and he's now playing a Hexblade who doesn't have fantastic damage but definitely has some great control elements.
What I found while he was alive was that Death attack was most useful against enemies with insubstantial as a quality. It can amount in an instant effective 20+ damage bonus, really helping to get rid of those creatures without needing to hit them again. Assassin's strike + sneak attack (Rogue MC) was pretty effective as well, but could only be used once per encounter. Once he blew his "load" (so to speak) his every day DPR was actually not very good. In fact I kept a reasonable track of the assassin in that game vs. when he was a thief at the start. In general, the thief did far more overall damage over the encounter, while the assassin had a better spike.
At the same time, the assassin's damage spike wasn't significantly better than the thieves. On an attack + AP the thief due to how reliable their sheer "every day" DPR is got really close to the assassin easily. Not to mention the thief is so ridiculously accurate that he hit far more often as well. If a fight went over a long period, the thief easily beat the executioner in total damage hands, feet and pretty much every other limb wise easily. It's hard for me to think the executioner is really underpowered though, because it has some really great features and assassin's strike (if used well) can be very handy. Overall though, I think a more reliable striker like a thief works better. Plus the executioner in that game was practically feat taxed venom hand master if he wanted to do jack with poisons, because many of their enemies are undead.
Also to keterys: Both his thief and assassin were optimized to make use of charging. As the game is dark sun many of the items to optimize it weren't there, but surprising charge certainly was. My other game has a Barbarian who almost certainly WILL be optimizing for charging, so saying it doesn't happen in a real game is a little incorrect. I have a charge optimized Slayer it would seem in my IRL game starting soon as well! Charging is just too efficient for many players not to actually do.