It's a great topic and I really liked this Lucky Paper Radio episode about it!
You can craft an environment where bombs they don't have a particularly high win rate. In Kaldheim draft, the bombiest creature, Koma, Cosmos Serpent, has only a 59.4% winrate when maindecked and a 71.5% winrate when drawn (data from 17lands.com). Note the average win rate of all cards there is ~54% (because the population who uploads data is better on average than the whole population), so it's more like a 56% winrate when maindecked and 67% when drawn. Definitely a bomb, but what feels bad about the card isn't its winrate, it's the lack of agency.
In situations where nothing can be done about the bomb, it feels like the game is pointless. Cube is a game where you draft a deck, build it, and play it. When someone opens a bomb, they just have to draft the bomb (and they renounce other cards for it, but it's fine because that is objectively the best pick), maindeck it (which does not require anything), draw it, and play it in not too precarious position. It's not a lot of effort, and when I play bombs I do not feel accomplished or happy or excited. It gave me no choice - I just did what I had to do, and if the opponent can't do anything about it, the game is over. If they can, the game continues.
If a card costs a lot more mana than one car usually spend in an environment, then even a balanced win rate for that card might feel bad. Let's say in a cube, Koma has a 60% drawn win rate (which is very reasonable being a 7-drop), with a 80% cast win rate.
30% of the time, you can't cast Koma and the other player wins, it makes you feel bad that you couldn't cast it and might make the other player happy that they closed to game fast enough that they dodged a bullet. There might have been choices involved, like the opponent playing more aggressively because they knew, but a lot of the time, it's just random.
20% of the time, you can't cast Koma but you win regardless. Koma feels like dead weight, and it reduced your options because it was uncastable.
40% of the time, you cast Koma and win the game. Some games with it might be interesting to you and to your opponent, but from your opponent's point of view, they are putting up a good fight against something broken. In other situations, it's just a thing that outright wins and gives you a huge payoff for simply reaching 7 mana.
10% of the time, you cast Koma, but your opponent wins. These can be really memorable experiences - either because they had to make spectacular plays to beat it, or because they had something even stronger, but if it's because they had exile-based removal, they had to use it and there were no choices involved.
My point is that in all cases, Koma usually has a negative impact on the overall gameplay experience, and the redeeming quality is when there is a fight over it. When your opponent keeps attacking and killing tokens, but they have to be way ahead for that to happen, and it is a very specific and rare board state.
Would you ever cube a spell that's 8BB Sorcery: Win the game? How about 5BB 6/6 Creature: When ~ attacks, you win the game? These are not very interesting, and we've been getting a ton of cards that say "win the game" with other words. Adding these cards requires you to either accept they are power outliers and will take over a lot of games, or have high power removal to balance them, which puts pressure on the environment, making interactions harder to assemble and narrowing the selection of playable cards in your cube.
Some cards don't outright win, but they might generate so much value and tempo advantage at the same time that they are almost always the objectively best play, and there's, again, no choice involved. Magic is more interesting when you have to balance your development of card advantage and board position. Cards that do both offer little choice.
When I played Angel of Invention in Kaladesh limited, though it offers a choice to the player casting it, it's often obvious what is the right play given the board state, and takes away a lot of choices from the opponent. The lifelink + vigilance makes it impossible to race (one less option), the flying often makes it impossible to block (one less option), the team buff make it hard to ignoring it and go around it, and the possibility of making tokens makes it a value play even against removal. It synergizes with a lot of stuff on the block and doesn't ask for anything extra mana in return. It's beatable with removal, yes, but there's no choice involved there. The opponent either has removal and uses it, or doesn't have removal.
In an environment where a 5-drop is hard to cast because most games end at less than that, it might be ok, but that's such a high tempo environment that your card and archetype choices are very limited and variance is exacerbated. Every play needs to be impactful, and there's little time for setup. Synergies have to run with the game to be worth it. That's more or less analogous to Modern.
