12 word passwords disgust me
May I ask, is this a personal preference, or is there a technical reason why this might not be a good idea?
Dictionaries can be collected, I think a private key is stronger, and I would like it longer, just as xmr's monstruosity.
Probably a personal preference yes but if you don't do them very well you might be more vulnerable for sure, they constantly being farmed and stockpiled.
Mnemonic code is as secure as private key. When you are generating 12 words mnemonic seed, the probability of getting same one is 1 in 2048
12. That is:
1 in 5444517870735015415413993718908291383296
I think that is secure enough against most of the brute forces. If that's not enough, you can even generate mnemonic seed up to 24 words. I prefer 15 words long mnemonic seed i.e. 160 bits of entropy and decrease probability of getting same one to:
1 in 401734511064747568885490523085290650630550748445698208825344
Well if that's not enough, you can always choose your custom passphrase to generate seed. Such passphrase will be used with mnemonic code to generate final seed which will be then used to generate private key and bitcoin address.
Well if that's not enough, you can be more creative. Choose two passphrases, let say 'mySecretPassphrase' and 'myNotSoSecretPassphrase'. Now after generating mnemonic code, create two seeds, one with each of the passphrase. Now send 0.001 BTC on the bitcoin address generated with 'myNotSoSecretPassphrase' + mnemonic and write down 'myNotSoSecretPassphrase' on the sheet where you wrote or printed the mnemonic code. Send your rest of the bitcoins on the address generated with 'mySecretPassphrase' but don't write it anywhere. Now even in the situation where someone got the hold of the sheet, they will be directed to the wallet having 0.001 BTC funds and no will even suspect that there is another wallet with the same mnemonic having all the funds.
In short, mnemonic seed is very secure way of storing and using bitcoins if you know how it works.