aoe boons decrease the per hit damage as it is being spread over more blocks.
I have seen negative durability (think it was a bug) on an iron hammer that had +1200 added. It counted to zero on the base durability counter then continued to count down to -1199 before finally being consumed on the last hit.
Also up until i think the 199 patch the bleed, slow, and weakness debuffs showed weird gui reference names. They now show properly from what ive seen.