I do know of at least two products that are shipping using YAFFS2, but I 
cannot divulge their names.

These have been shipping for a while (over a year) in reasonable volume. 
Niether of them use hard links or mtd.

Recently a problem was found in hardlink scanning which could cause a crash 
during mount. This problem was addressed in yaffs_guts.c 1.26, over a month 
ago. I have not heard any feedback that this is a problem.

The much-discussed mtd hook-up seems to be the biggest single area causing 
problems right now.