Yeah, the topic really isn't about which card is unplayable regardless of situation, but rather just which one you personally wouldn't play for whatever reason you have. Like me for example. The card Pot of Desires, is probably one I would never play unless my deck benefited directly from the banish cost, such as a Gren Maju deck. In other decks, I wouldn't ever play it because the cost is too great, not only are they banished, but the face down banishment is what takes it overboard for me. But, I've seen many other decks that don't benefit from the cost, use it anyway. I just feel like you've got a really good chance of throwing away cards you will need. A 4th of your deck everytime you play one and I've seen decks run 3 of them. Imagine burning 3/4 of your deck hoping the last ten cards will win it for you. Too much in my opinion.
I kinda wish they'd bring back Yata and Envoy of the End. :p
That's not how desires is supposed to work. You don't activate 3 desires, basically ever. Most decks can't resolve that many, because each shaves 12 cards off of your deck, 36 in total, along with the opening hand of 5, and 2 additional draws because of HOPT, you need 43 cards in deck minimum, under the most ideal circumstances, assuming you use NO other cards from the deck. You usually only want to activate 1, 2 if you're desperate, but never 3. People play 3 to see it more often, and earlier in the game, because most of the time, the banish 10 doesn't matter, and the +1 is too good to pass up. The second desires is a gamble, and the third is a dead card. It's like the 3 brilliant fusion, 1 garnet idea. You wanna only resolve it once, but you play 3 to see it sooner.
Thing is, cards in the deck are cards you usually never see, because games nowdays last less than 5 turns, unless both players are garbage or one of them plays psy-frames. Desires is a good card under most situations. There are some decks that can't play desires though, such as zoo (Though the deck's dead), Frog decks that depend heavily on the frog engine (The more paleo heavy ones can have a field day with the thing), and other such combo heavy strategies (World chalice maybe? Never played the deck. Spyrals, too, I would imagine, though I can't be sure, and maybe some BA builds, because their three best main deck cards are at 1.), so you shouldn't splash the card into everything, but for most decks, it's a pot of greed with only a mild drawback. And if you don't wanna see dead cards, you can always just play 2, or 1, even.
All that said, Light of sekka, on the other hand...
I'm going to stick with my previous statement and say that any cost is justifiable as long as it has a good enough payoff. Cards like beginning of the end aren't played, not because they have a large cost, but because they're too situational, and 7 darks is a lot to ask.