Can I restore a wallet using any valid 24 words? Can I create a wallet and restore it on something like a Ledger S using any 24 words that I choose from the list of accepted words?

Is there any disadvantage to creating a wallet this way instead of having say Ledger generate 24 words for me?
If you choose the words yourself, you're more likely to be biased towards some word selections based on position in the list, words you like and dislike, etc. Ledger is designed to pick the words as securely as possible. Also, some words are used as checksum I believe, so if you just picked them randomly the checksum would be invalid and you'd probably get a warning. But yes, it would still probably work.