I wouldn't trust my mind to store seeds...
Right, even at the very least that you are something exceptional about it, don't trust your self about memorizing things especially words, writing it the very best way, then if you have something more to buy, use a back up steel works or something similar that is fire and water proof, that, at least the best way.
The mind cannot remember something important forever, especially a very valuable wallet seed, forgetting just one letter means you can't access your wallet. Even though your memory is very sharp, your memory will fade with age, or maybe there is something we don't want that makes our brains hit, of course this will make some of our memories disappear.
So memorizing the wallet seed is not a solution, it's better to write it down in a place that is waterproof and fireproof. Even if you want to memorize it, it won't be a problem, just think of it as learning to sharpen your memory, but keep in mind that you need to have backup storage. Don't rely on your memory to store something very valuable because it will make you regret it later.
Thats definitely the main risks on which you do need to look upon because if you would be that forgetting even on a single word then you're fucked up. Come to think that whenever that new things that would instill into your minds then those older things or thoughts will definitely be having that chance or tendency for it to be forgotten on which simply means that its not something safe but since its been mentioned that he had made out some back ups with other means like on writing on a paper or having hardware wallet then it wont be that much of a bothersome if you will be able to forget but its not that bad to have multiple back ups including with that memorization but just like on what most people been saying on here is that you wont be able to rely on this one if we do speak about on sole memorizing those keys on which it will be that so hard into this aspect if you would be having this kind of kind of storing those seeds.