@JeremyECrawford Curse of Strahd: Champion Fighter accepts a dark gift, it lets him cast Contagion (it requires a melee spell attack). The Champion doesn't have any Spellcasting Ability (Charms, on DMG, don't cover this).
Which modifier must use a Champion to hit with Contagion?— Draconis (@DerynDraconis) June 7, 2018
Sometimes a non-spellcaster gains the ability to cast a spell. If you're in that situation and a rule doesn't tell you how to calculate your spell's save DC, use the default calculation in the spellcasting rules (PH, 205), and your spellcasting ability modifier is 0. #DnD https://t.co/F4WYUluw7h
— Jeremy Crawford (@JeremyECrawford) June 11, 2018
So… according to Crawford, for a Fighter, it’s essentially useless.
It’s pretty standard to assume a Fighter enjoys at least a +3 attribute modifier from either Strength or Dexterity, and also gets to add their proficiency bonus, which starts at +2 and only increases from there as they level.
So they go from having at least +5 to hit on their attacks, to having absolutely no bonus to hit at all. Or in other words, they suffer an effective -5 to hit, at minimum. More realistically, it’ll could easily be a -8 to hit by 6th level, and it only gets worse the higher your attribute and proficiency bonuses get.
A -5 to hit is massive. It is generally on par with having Disadvantage. A -8 to hit is just staggeringly bad.
Now, to be fair, the truth isn’t that you have a minus to hit – you’re simply stuck with just a D20 roll. But that’s plenty bad enough.
If you roll only a D20, with no positive modifiers, you have only a 55% chance to hit a target without any armor bonuses at all.
An enemy with a AC of only 15 would leave you with only a 30% chance to hit. One with an AC of 18 would risk only a 10% chance of being hit.
Nine times out of ten, a Fighter using this power against such a target would utterly fail. That’s not a Dark Gift. That’s a false promise, and bad game design.
In contrast, the same fighter given the minimum reasonable attribute bonus and proficiency bonus would have a full 80% chance to hit an unarmored opponent, a 45% chance to hit one with medium armor at AC 15, and a still useful 35% chance to land a blow on the heavily armored one with AC 18.
And on top of that, normally Fighters make use of and rely on their ability to make multiple attacks per round, further increasing the odds of actually accomplishing meaningful combat progress on their turns.
A power that uses your action and has only a 10% chance of succeeding (and that the enemy can still make a CON save to negate!) is simply not worth using, as you will almost always accomplish nothing and waste your turn.