The benefits is you don't have to worry about storing your shit elsewhere, you just have to keep it in your mind, and best thing about it, no risks of some hacker being able to get a hold of it, or some thief smart enough to be able to access it using the recovery phrase.
No hacker can hack your brain but is this advantage big enough to ignore disadvantage?
The cons or the setbacks is of course, the unreliability of the human memory. Shit you can remember for today, you may not be able to remember in the next couple of weeks or so.
Human memory can be lost anytime and it's different than storing seed phrase in digital devices. We can have different disks, USB sticks for back up but with memory, each person has only one brain to use. No other reserved brain to make a second or third brain, memory backup.
This lack of extra backups means higher risk to lose our bitcoin forever when our brain and memory have problems.
If you are going to memorize your seed words at least have a paper backup around because you cant know when the dementia will get you. It could be anytime. It happens to young people too.
It only takes one forgotten word or letter to mess everything up. Bitcoins history is full of stories like that.
I don't want my story will be added to stories like losing bitcoin because lack of back ups. I don't feel anything funny to read stories of people lost their bitcoins but if it is my story, I will feel very painful.
Relying on brain and memory is a best method to lose bitcoin and with this big risk, I don't choose this method.