Why are some decks (i.e. Meandeck Tendrils) not playable in tournaments?
Effectively, you need to be able to win two games (possibly after losing one!) in 50 minutes, and give your opponent their fair share of that time too, so if a deck is difficult and time consuming enough to play, you may simply run out of time, and not be able to win matches. And you may never even reach the time limit - if a judge thinks you're playing slowly enough to cause problems, they will step in and force you to speed up or lose. If on the other hand you play fast enough to avoid all this, you'll make too many mistakes, and lose anyway.

All these things are potential issues since as Menendian says in the article, a lot of the decisions you have to make are difficult, and the deck is difficult to play fast enough without making mistakes. (Of course, it's also possible that you can play it fast and well even though he couldn't - feel free to try to prove him wrong!)



In tournaments, the time limit is typically 50 minutes per round. (Officially, the minimum time limit allowed is 40 minutes but 50 minutes is the recommended time for constructed tournaments). In each round you're playing a best of three match. If someone hasn't won two games by the end of time, the current turn is finished, then five more turns of play are allowed (i.e. three for one player and two for the other); if the game isn't over by then, that game is a draw. If that was the third game (reasonably likely, it's been 50 minutes), each player has won one of the first two games, so the match is a draw as well. So if a deck is slow to play, you'll likely end up with a lot of draws.

In order to help make sure that people actually finish their matches, the tournament rules disallow "Slow Play". It's normally something where a judge will simply give a caution, letting a player know they need to speed up, but if it continues to happen it can lead to more significant penalties. You can't just sit there thinking forever; you'll eventually get a game/match loss.

The point at which you're playing too slowly is up to the judges' discretion, though. This article by a couple of judges goes into some detail about the way they personally think about it, though it's explicitly not official policy. The key thing to notice is that it's largely based on what speed of play is necessary in order to finish games within the allotted time. So while for many decisions 15-30 seconds is about as much time as you can reasonably take, if you're to the point where clearly you'll either win or lose within a minute once you decide, you might get some leeway. Bottom line, though, it's possible you won't actually get to the point of running out of time in the round: a judge may step in sooner and force you to either decide (making your decisions more error-prone) or lose.

There's another problem on top of this: combo decks tend to have long turns, so those few extra turns at the end of the game can drag on for much longer than with other decks. There's only a small amount of buffer time between rounds, so the two players in the extra long match will end up delaying the next round, upsetting the schedule for the entire tournament. Wizards actually banned Second Sunrise from Modern in order to stop a combo deck from causing these kinds of delays. That doesn't exactly mean the deck was unplayable (in fact people were doing reasonably well with it), but at the same time, you don't exactly want to be messing up tournaments for others.

As for this specific deck, I can't personally tell exactly how difficult and slow it is to play, but it certainly looks hard. There are an awful lot of decisions, and getting any of them wrong could totally derail you, so you'd have to take your time and get it all right under pressure. I would tend to trust the opinion of the experienced player who wrote that article and assume he's right that it's simply too hard to play within the available time. On the other hand, perhaps with enough skill (or enough practice) you could get fast enough.