If someone is using Great Weapon Master or Sharpshooter, they need to be able to calculate when to use it. It’s really not hard to do so on the fly though, just ask yourself: "What percentage of my damage is (GWM / Sharpshooter)? "
If it’s half your damage, you can halve your hits before it’s disadvantageous. Note this is always in comparison to your rate with it off. If off you’d hit 60% of the time and on you’d hit 35%, It’s still worth using, lower and it’s not, you’d be nerfing your own damage by leaving it on.
If it’s 1/3 of your damage, you can drop your hit percentage by 1/3. Going from 90 to 65 is fine, lower and leaving it on is worse.
If it’s 1/4 of your damage, you’ll basically only use it when you’re dropping by 1/4 of your hit chance or less, which means either enemies with low ACs where your accuracy was overkill beforehand, or against enemies where you have advantage.
https://anydice.com/program/324ecThe link above is dice probabilities, with advantage, normally, and with disadvantage. Since you always hit if you get "at least" the target AC, the "At least" tab will always be directly reflective of the probabilities you'll see in game. Thankfully you don't need to manually calculate them, if you have a +10 and the AC is 15, and you have advantage, it'll just show 96%, or if you have disadvantage, 64% (or if neither 80%). But it gives you an idea to the kind of space where Sharpshooter and GWM will still be worth using even if your initial damage is 30 or more. If you're going from a 2 to a 7 with advantage (99.75 to 91), you could have 80 base damage, and GWM / Sharpshooter will still be worth using.
So as long as you understand your odds, and understand when to use it, it can be a tremendous boon. Often it will increase your expected damage output by 50%+ in the early game, and continuing to pay dividends throughout the game. If someone hasn't already, they should try pairing someone with GWM and / or someone with Sharpshooter with a Druid who makes effective use of their spider form and web, it makes the early game an absolute slaughter.
Later on, you can get reliable sources of advantage with items, the most famous being the Risky Ring, but multiple options are available. You also gain access to many types of paralysis or sleep effects, at which point your hit percentage is always 100% and more damage is always better. Note that this 100% hit auto crit mechanic even applies to ranged weapons within 1.5m, and they both auto hit and auto crit without needing Crossbow Expert to negate the normal disadvantage they would have for their target being prone if they were conscious.
Finally, it should be noted that the ancillary benefits will apply regardless of your use of the passive. +2 to hit when shooting at enemies with high ground over you and a bonus action attack on a crit or kill remain useful, potent bonuses at all stages of the game, even if you never have the passive on. But, if you use advantage and CC well, and can handle basic math calculations, you'll certainly have it on more often than not, and as a result you'll do some very silly amounts of damage.
