BIP-39: Add shortcomings section, drop comments headers.

Author: ajtowns

bip-0039.mediawiki

@@ -6,8 +6,6 @@
            Pavol Rusnak <stick@satoshilabs.com>
            Aaron Voisine <voisine@gmail.com>
            Sean Bowe <ewillbefull@gmail.com>
-  Comments-Summary: Unanimously Discourage for implementation
-  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0039
   Status: Deployed
   Type: Specification
   Assigned: 2013-09-10
@@ -138,6 +136,18 @@ Also see https://github.com/bip32JP/bip32JP.github.io/blob/master/test_JP_BIP39.
 
 (Japanese wordlist test with heavily normalized symbols as passphrase)
 
+==Shortcomings==
+
+Some shortcomings have been identified with this proposal:
+
+* Generated seed depends on the wordlist that was used for the mnemonic. Because the "mnemonic to seed" process uses the mnemonic sentence directly rather than the original entropy, translating the mnemonic to a different wordlist necessarily creates a completely different seed. This is not an issue if you only support the English wordlist, as recommended above.
+
+* Because the seed is generated by hashing the mnemonic, it is not possible to represent an arbitrary BIP-0032 seed via a BIP-0039 sentence: the conversion is one-way only (from BIP-0039 sentence to BIP-0032 seed).
+
+* The checksum is short. This means it only gives modest odds of catching random errors (1-in-256 errors will be missed). It is also not able to provide any assistance in correcting errors.
+
+* No versioning scheme. When originally introduced, there was no way to distinguish the address format that should be used for a BIP-0039 key. This is now largely mitigated by use of descriptor wallets (BIP-0380) in addition to a seed however.
+
 ==Reference Implementation==
 
 Reference implementation including wordlists is available from