.. and exposing lots of buggy userspace code into the bargain!

What kind of bugs appear with bigger default page sizes?

Lots of userspace makes assumptions about page size being 4k and breaks when it changes. Try looking for:

https://www.google.co.uk/search?q="pagesize"+"64k"+"bug"

https://www.google.co.uk/search?q="pagesize"+"64k"+"issue"

Another common one is actually in the kernel where filesystem block sizes are limited to page sizes, so from this point of view large page sizes are better:

https://lwn.net/Articles/591690/

And an even slower page fault mechanism. PPC generally ends up relying on hugepages to try to match pagefault speeds on x86