Anything can fail -- but overall, I believe Steripens are dependable. I used my Adventurer regularly on a 7-month RTW trip last year without any problems whatsoever.
To me, I think the decision between Steripen vs. gravity filter should center on water sources. If they are fairly clear and good to OK tasting, then get the Steripen. For brown river water, green algae ponds, or any water with lots of sediments, larvae, dead bugs... I will definitely want a filter that will clarify water and improve its taste.
I find myself using a combination of chlorine and filter when out in the wilds -- and using the Steripen on world travels (hosteling) -- where tap water usually comes out clear and OK tasting -- but biologically suspect.