restoring from a mnemonic requires selecting the correct mnemonic type, i.e. won't let you proceed without it. it may be safe to automatically recognize and select the type for correct seeds, not least due to the distinct number of words for each. this could make wallet restoration more effortless.
one reason to not do this could be that, while entering a mnemonic, it could yield a correct mnemonic for another shorter type before being fully entered. I'm quite sure that there's a negligible chance of the first 16 words of any legacy mnemonic being a valid Polyseed, but I don't know how MyMonero mnemonics are encoded.
restoring from a mnemonic requires selecting the correct mnemonic type, i.e. won't let you proceed without it. it may be safe to automatically recognize and select the type for correct seeds, not least due to the distinct number of words for each. this could make wallet restoration more effortless.
one reason to not do this could be that, while entering a mnemonic, it could yield a correct mnemonic for another shorter type before being fully entered. I'm quite sure that there's a negligible chance of the first 16 words of any legacy mnemonic being a valid Polyseed, but I don't know how MyMonero mnemonics are encoded.