Hey there. I'm a current rager who has been ranking heavily on invoker/conjurer mobs, as well as having a few long, drawn out fights with invoker PCs.
I've only seen spellbane go off once or twice in about 30 hours of being a rager. I could be crazy, or slightly biased. I remember a few years ago I was playing a rager and asked about spellbane only to be told all was fine, and then a few weeks after I deleted it came out that there was an issue with it not working.