I do that too, Lys. I've made one deck with 24 lands (ramp, where it's really needed.) Everything else is 20. I still have games where I flood out, but I also have games where I'm starved. That's a card game for you, though. There's always a little bit of randomness you can't get rid of.
Just a note - 20 land is very light.. Like, most aggressive decks (highest mana card being 3, a 1 or 2-of) run 20 land at minimum.
I agree that 20 lands is very light - it is not just about average cmc and curve. Always consider probability as well. Even if a deck is monocolored, only plays untap lands, and "only needs 3 lands", when you play 20, you will only get 3 lands 43% of the time in your opening 7 cards (27% = exactly 3, 16% = more than 3, and 4 or more lands in opening 7 would probably be considered bad in a deck that only wants 20 lands total).
That means over half of the opening hands in a 20 land deck would have 2 or fewer lands. With only 33% of the deck being land, statistically speaking, one should only expect one land for every 3 draws. That is not digging out of a mana hole very well.
So, mulligan to 6 on opening hands with only 2 lands in a 20 land deck? The chances of getting 3 lands with 6 cards drops to 31%.
If you are ok with starting at 2 lands and banking on drawing more, 20 lands could be the right number. But math says add a land or two if you want to avoid having to mulligan to get the right amount of land to start a game.
Everybody is different
Makes the game fun.