I think the big difference between people benefiting at small doses (~0.3mg) and large doses (2+mg) is that the 0.3mg group use it for sleep quality through the night, whereas the 3+mg people just need the sudden shock to get to sleep in the first place.

The drawback with big doses is that your brain becomes less sensitive so your naturally-produced melatonin might not be enough to keep you asleep for the whole night after the pill wears off. It has a very short half-life in the body (under 1 hour), so there's no way for a single dose before sleeping to last 8 hours. We naturally produce only 0.06-0.08mg per night, so it's easy to see how supplementing melatonin could desensitize someone and cause them to wake up after just 4-6 hours of sleep.

I have ADHD and am in the large-dose category and use 2-3mg of melatonin to help me fall asleep. Without it, I can't sleep reliably because my brain often won't shut up. Sleep reliably is so much more important to me than sleep quality.

Using it only 5 nights a week, I'm not significantly dependent. I can still sleep without melatonin, just less reliably. I've tried 0.3mg, but it felt the same as taking nothing.

For me, 10mg would be excessive and probably harmful in a desensitizing way. The most I've taken is 6mg, but it only helped in 2 out of 6 times. The other 4 times my brain just wouldn't stop. If doubling my usual dose didn't help, I don't think doubling it again would be any different.

There are however studies with higher doses, e.g. this one about kids with ADHD that says:

two-third of the patients responded to relatively medium doses (2.5–6 mg/d), whereas doses above 6 mg added further benefit only in a small percentage of children.

so I guess it's different for everyone.

Since it's a natural hormone the body already has ways to get rid of it. It has a half-life of less than an hour. The lethal dose is so high we haven't been able to intentionally kill animals with it: "Melatonin is not fatal even at a dose of 800 mg/kg in animal studies".

The big risk of ongoing high doses is becoming so dependent on it that you wake up as soon as it wears off (e.g. after only 4 hours of sleep). At this level you basically can't sleep without it and have to slowly wean yourself off to get back to normality.