It looks like going stealth regardless of your ambusher level does not seem to affect undead, night creatures or bogeymen , their sight is 360 degree apparently , and considering they're all hitting extremely hard, it's a good idea to avoid fighting alone
About the companions, they do not seem to use stealth themselves, so if you're trying to sneak somewhere, they will move next to you and attract the attention of everyone (prepare to change your speed to Sprint and run as fast and far as possible in case you see plenty of undead near).
"fortunately" when it's about bandits it's not that bad to be spotted because of your companions, relationships are a bit bugged and most of the times bandits will not be hostile (nothing to do with the challenging bandits of 34.11) , and will even greet you as if they were your best friends, and it's so badly broken that killing bandits will make your castle leaders to hate you and call you murderer, even if they were the ones that told you their city was harrassed by bandits.
Since a version of two a fix was done to stealth , so when you hit someone while stealth is on, it will react assuming you have not 1 hit killed him, previously when stealth was on you could destroy a whole civilisation by yourself as none reacted to being hit.
Now none will react to their, even best, friends being killed in their sight by you when you're sneaking, hopefully fixed soon.