You've either partied up with very bad WHMS, or haven't fought against a good one.
WHMs are extremely tanky if they take advantage of the LoS. I've fought against WHMs that can literally tank all other 3 members focusing it, manage to heal their group, repose, raise any KO'd group members. I've fought 10 minute battles because they had an extremely good WHM.
So WHMs dont suck, people suck. L2P
