Edit: A lot of people say, that GWM needs a melee weapon attack, but they miss Jesses point: While GWM requires a melee attack with a heavy weapon, Sharpshooters only criteria is an attack with a ranged weapon (not a ranged weapon attack). Jesse bases his claim on the fact, that a crossbow is still a ranged weapon, even if used as an improvised weapon for melee combat. That’s why it deals 1d4(!)+20 damage. (It works with any ranged, heavy weapon btw., so Longbow qualifies too.) Of course Jesse is playing the devils advocate here and of course, no somewhat sane Walter will allow this in any campaign ever, as it’s obviously not the intention behind these feats. But you could read it that way and that’s Jesses (paperthin) point. Besides: he finds the image of a barbarian running around recklessly smashing a crossbow over everyone’s head to just be hilarious.
The flavour text for the feats doesn't actually give the details of what rules they have. That's the issue- the rules are vague, rely on other bits of context which may or may not be considered part of the rules depending on the reader, and are followed up by a series of twitter rulings which tend to muddy things more often than they clear them up. In 5e, RAW, you can bonk people with a crossbow and use the third point on the sharpshooter feat- same as how in 3.5e, RAW, a chicken infested commoner could pull infinite chickens out of their spell component pouch, or an iron heart surge could take out an anti magic field, or drowning could heal someone from -1000HP back to 0. The point isn't "Hey, look at these things you should totally do in the game", the point is "here's what the rules literally say". And, by a literal reading of the rules, the sentence "Before you make an attack with a ranged weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack's damage." must stand on it's own.
'Flavor text' is context because it shows that the text is describing. If a reading of the rules contradicts the flavor text because it is considered unclear, then the reading of the rules that contradicts the flavor text is wrong.
I'd be inclined to agree with you if there wasn't such a specific divide between the flavour text and the content. For instance, the sharpshooter feat doesn't specifically let you make shots that others think are impossible- saying "I use my sharpshooter feat to shoot the BBEG in his castle three hundred miles away" would certainly be a shot that others think are impossible, but I doubt any table would let that fly. Saying "I have the crusher feat, so I'm going to break the enemy's bones to debuff them" would fit directly into the flavour of "You are practiced in the art of crushing your enemies", but it's also not what the feat does. For the halflings, you can take the bountiful luck feat, and then use it, despite the flavour text clearly stating "You're not sure how you do it; you just wish it, and it happens"- thus precluding anyone who knows about the feat or what it does from taking it. Lucky: "You have inexplicable luck that seems to kick in at just the right moment.", but it's actually very explicable, follows predictable rules and doesn't kick in at just the right moment unless the player knows just the right moment to use it.