Whats an example of this failing?

in this particular case it was ozone, when userIdAsEids are undefined, the iteration in findAllUserIdsFromEids https://github.com/prebid/Prebid.js/blob/master/modules/ozoneBidAdapter.js#L682 fails because they only check if the bid-request object has a property userIdAsEids (which was likely the old behavior that this would only be set if their were user-ids), but don't check if the property has actually a value/is iterable in this if; https://github.com/prebid/Prebid.js/blob/master/modules/ozoneBidAdapter.js#L677
as this function is called while constructing bid-request, the bidder errors out, and is not able to perform the bid-requests.
But i wouldn't be surprised if more bidders or consumers of the userId module would follow a similar access pattern. hence the fallback to an empty array. of course it would be good if the bid adapter it self would also check if it can iterate over the collection.