A lot of combos are like this: they'll win spectacularly, but only if you can play four different cards on the same turn that require three different colors and no counterspells from your opponent. Guess the odds on that actually happening.
Spawnsire of Ulamog's ability lets you play as many of the humongous Eldrazi cards as you want, right now, for no extra cost, and without even having to have them in your deck... if you can somehow get the whopping twenty mana it takes to activate it. It wouldn't be terrible, except that having all of your Eldrazi at once is almost always overkill—for just over half the mana cost as the Spawnsire of Ulamog, you could just cast Ulamog itself and skip the middleman.
Any of the legends from Legends could be considered.
Magic's early card designers didn't appreciate that giving a big creature a high mana cost was already a drawback, hence why cards like the elder dragons, Polar Kraken, Leviathan and Marjhan have such hideous drawbacks in addition to their huge cost.
Much like the Elder Dragons, Nicol Bolas, Planeswalker. His +3 ability is to utterly destroy any non-creature card in the game (in addition to providing an obscene boost to his Loyalty total), and his ultimate effectively left your opponent topdecking with very little resources. The catch? You need 8 mana, 4 of which were split into three different colors. That's never been an unattainable feat, but the same block had Nicol Bolas' signature spell Cruel Ultimatum in the same colors for one less mana that would almost always win the game when it was cast; Nicol Bolas, while still a kitchen table favorite to this day, was left on the sidelines as all the competitive decks opted for the seven-mana sorcery instead.
Darksteel Reactor and Helix Pinnacle are big, flashy, nigh-unkillable instant-win spells, but most of the time, the game is already over long before they finish charging, and if it goes that long, you could probably win just as easily with any generic flying creature.
Also, most instant-win cards only activate if their condition is true at the start of your turn, which means your opponent usually has a full turn to either destroy the card, invalidate the condition or finish you off (unless you can somehow create the right condition at instant speed just before the end of their turn).
Triggering the ability of Door to Nothingness can be especially difficult. You have to have two mana of each of the five colors to trigger its ability, which is far from trivial to achieve. You also have to leave it sitting on the table for at least one full turn and hope it doesn't get destroyed. However, if you do succeed in this, the target player loses the game on the spot.
The Epic spells from Saviors of Kamigawa are five spells with awesome, flashy effects that repeat themselves every turn for the rest of the game after you cast them the first time. The drawback is that you can cast the Epic spell every turn, but nothing else—you can't cast any new spells. Enduring Ideal, the white one, did break out into the tournament scene for a short time, but the other four never caught on.
The card Chalice of Life/Chalice of Death can kill an opponent in four turns—assuming you can keep your life from going to zero AND keep the card on the field long enough to transform it, AND keep the opponent from gaining life, not to mention the fact that you have to wait for your life to get to THIRTY just to use it...
The B. F. M. (Big Furry Monster) from the joke set Unglued: BBBBBBBBBBBBBBB * that's 15 black mana! is a huge cost, even for a 99/99; most games would be wrapping up by the time you got that kind of mana. Since each half is useless on its own, Victimize is the only way to get it into play via the graveyard, and it has a ruling explicitly stating this is allowed. Since Wizards generally doesn't issue rulings or clarifications on the Un- sets, it's not clear if something like Fist of Suns would let you play B. F. M., since it's two separate cards.
Phage the Untouchable got a lot of attention when she first debuted due to her abilities, but she was such a hassle to use that she got the name Phage the Unplayable. Opponents will probably have much larger creatures by the time you have 3BBBB handy to play her, combos to get her into play from the graveyard or library are very complicated and easy to spot, and she's still vulnerable to counterspells, blocking, instant-kill spells, and so on.