I purchased a r6ii with 24-105 f/4 L Nov 2023 and I've had it fail on me at weddings three times now with respect to freezing and/or memory card corruption. I've also had it freeze/lock up in recreational shooting - but the stakes are obviously lower.
I always shoot with two memory card slots, raw (not compressed) + jpeg on each (aka record multiple). I was never using burst mode of any kind in the failures A, B, C.
Failure A: the camera freezes, and turning the off switch doesn't unfreeze. I have to take battery out and put back in. Card is fine and camera is fine.
Failure B: The buffer in one of the memory cards kept writing out fragments _of the same picture_ every time I took a photo. If I loaded up the photo on my laptop (unreadable on the camera), I could see some percentage of the photo but it was always the same one. Fortunately the other card slot got the new correct photo with every shutter click.
Failure C: Camera locked up. Said card 1 was unreadable and needed to be reformatted and wouldn't take any pictures. Powered off and had to remove battery, and on, same error. Powered off, removed card 1, powered on, and finished shooting fine on slot 2. Card 1 was unreadable in camera and unreadable in laptop- card was recognized when put into laptop but would not mount.
General lock ups happen with Sandisk extreme pro 256 GB cards (two different cards, same type UHS1 V30 purchased at the same time from Amazon) and also the Kingston 128 GB UHS II v90 128 GB card. I've not had any lock ups or problems with the Kingston in my canon rebel sl2.
I've had problems with firmware before update and after update to 1.2.0. The freezes have happened with the canon rf 24-105 lens and the canon rf 100-300. I always reformat the cards before use using low level format in the camera.
I have not had problems with using the 256 GB cards for video (but I only shoot a few dozen video clips vs thousands of photos). I have never had any freezes or lockups using a sandisk ultra 64 GB (UHS 1 sd speed class 10- cheapest possible) or a sandisk extreme 64 GB card (v30 uhs1) or lexar cards <= 64 GB or a san disk card that is 32 GB.
Update 2023-11-11
Failure 😧 I took the 256 GB card that failed on me with no recoverable images, put in camera slot 1, reformatted with low level format.
JPG + Raw (not craw), record multiple - but _no_ card in card slot 2.
Could take pictures fine.
Set to electronic shutter (all previous failures were with the mechanical + electronic shutter at weddings because I need flash to work).
Canon brand battery (2 bars charge).
Burst mode H (not H+ and not low speed). All previous failures were in single shot servo mode.
I squeezed the shutter button to take pictures of a spinning golf ball in daylight. I was able to record to fill up the buffer, it would stop taking pictures, I'd let it drain the buffer back to full capacity, and then repeat. I repeated this sequence about three times and after the first or second time, the buffer took perceptibly longer to empty/reset. At the fourth time, I looked at the camera while it was clearing the buffer (not taking pictures), and saw the Error 02 code. "Card cannot be accessed. Reinsert/change card or format card with camera." Turning off the camera did not clear the error message. I had to physically remove the battery (this is 'normal' for me, but seems like another bug). However, now the card does not even fail to mount with an error message in my laptop- no sign it is recognized. I put the card in my r6ii and also my sl2 and neither camera is able to reformat the card.
The camera is under warranty until Nov 30, 2023.
