Thanks!

The mention was at about 12:06, in the form that OLM breaks down at about 50 users "give or take", so it's not really a limitation imposed by the system itself and it would be difficult to impose it. I doubt this is the experience of all Matrix e2ee users at least at that exact point, but e2ee has always had some growth pains, so there could people with those issues; on the other hand few large rooms are e2ee to begin with, so experience on those is limited. E2ee also requires the users to be more mindful about their data as in not to lose their private keys, and these problems probably increase linearly as the room size increases.

I didn't notice any claim of rooms larger than 50 becoming public.

I've only heard a second-hand info about it, but apparently one local policital party uses e2ee in Matrix with hundreds of people in the room, so that should be a proof that the encryption is not limited to 50 users—and this info sounds just as well founded as the information provided by the video ;).

The guy carries on stating that pretty much all of the huge matrix rooms are not end-to-end-encrypted, and I have no reason to doubt that. Personally I see little point in having such large rooms encrypted anyway, because if you have a large room you will also likely have very relaxed checks on who gets to enter it (e.g. it could be completely public), and if that's the case, then so can any party who wishes to monitor the room join the room as well. E2ee won't be protecting those cases. (While at the same time you lose server-side search feature and efficient notifications, though at least the latter one is being fixed with out-of-envelope notification data—which again leaks a bit more metadata..)

The video also makes it sound like that if you have a Matrix Home Server in the network, it's going to end up hosting CSAM. This is only the case if one of the users of that HS are in a room that has the content, so it's not like it will just automatically get migrated there. I imagine vast majority of Matrix Home Servers have limited account creation abilities (e.g. companies, personal home servers, organizations, etc), eliminating or at least highly discouraging this kind of issue.

Btw, the video makes an excellent point about the Matrix CDN issue, which is being fixed currently as well (that change is already merged to the matrix spec), by requiring authentication. Next steps is going to associate media to messages, making this kind of thing even more strict. All this means IRC bridges will need to start hosting Matrix-side contents by themselves, though..