This.
Your flush is probably good against whoever is making the $20 bet. It's the number of other players in the pot that would be the main concern.
Your actual odds to hit the flush are 1 - ((1 - 9/47) * (1 - 9/46)) = 34.9%, much closer to the 33.3% pot odds you're getting than your estimated 36%
1.1 > 1.6 ?
There's not much risk here, he's all in for $20. Either he draws out and wins the pot, or he busts. Personally I don't play any 2 suited, but if you do, why would you shut down when you "hit" the flop?
You guys have mentioned a couple reasons; firstly, if the board is paired and the OP is up against a set, his winning odds are reduced to about 25%. But does someone who flopped a set bet the pot to protect his hand? That depends on the bettor and the rest of the players in the hand.
As for facing a bigger flush draw, that's certainly possible depending on the table, but I think the statistical odds aren't very high even for a full table.
The way the OP estimated his winning odds is perfectly fine, # outs x 4 on the flop or # outs x 2 on the turn. It's way easier than the precise methods that are impractical at the table. The real question is if he has 9 full outs, fewer in the couple bad scenarios, or additional backdoor outs if he's up against just a pair.