Ideally the shifter will move a little less than 90 degrees. If you can physically get it to move more than that I would say the cable tension is way off. When you say "caused the cable to fail", do you mean caused the cable to break? That would be unheard of, unless it was severly corroded. Even cheap brake cables are incredibly strong and the stress put on them isn't much to move a derailleur out. Something else could be wrong here. Start with a new cable and adjust the initial tension as per the Park Tools website or any of the other online sources.
Bottom line is that it isn't the shifter, I'd say, but something else. The shifter itself is a fairly "no-brainer" device.