It's hard to quantify when it would fail, but the problem with too many fields is the reduction value keeps growing unbounded and it's stored in a inner node in the btree. Then it becomes a problem of only being able to store 2 reduce values in the node and parsing and computing on these big structures every time the view is queried or updated, and slows everything to a crawl. If it gets too big, the js engine will timeout or run out of memory.
I believe the best advice is to not let the total size of a reduce value, when serialized to text, be larger than 1k. If it's larger than that, you are probably doing something wrong.
Can we please:
-Investigate and reproduce the errors associated with http://www.couchbase.com/issues/browse/CBSE-483
-Create and resolve any necessary bugs
-Document the limitations and provide best practices to avoid this in the future