It looks like I have just straight up been playing them wrong. I should have been using them as a mobile fire-base/harrasment unit instead of just charging willy-nilly with them.
My current plan is going to be to model up a nice fat and juicy Icon of Nurgle for my bikers, give em a few meltaguns, and try again and see how they do.
Depends on what you want them for. I would go anti tank or anti infantry, not both. Now-a-days, antitank seems to be the favorite. They are also good for termies, oblits and demons to drop next using an icon. Since they are few in number, they should always be part of 1-2 or 1-2-3 punch.
If I remember right, when you first started running them you tended to always turbo boost them and then charge them in on the 2nd turn - sort of leading ahead of the bulk of your army. It always gave your opponent the ability to pick them apart since they were the lone threatening target that arrived on the 2nd turn.
As much melta as possible and mark of Nurgle. Run them up a flank and have enough teeth in your army to make them a lesser concern for the enemy or enough of a concern that they take the heat off a critical unit.
If Wildeyedjester's comment is true, your problem is that you're rushing them into melee. Close combat should be a last resort for a Chaos Biker. They are no more effective in melee than a normal CSM, yet more than twice as expensive. Their main strength lies in their maneuverability, wargear options (mostly meltaguns) and the Twin-Linked Bolter. Flank and hit vehicles side/rear armour, then focus on your opponents' elites. Stay out of charge distance, since your Bikers don't follow "vehicle" rules, you can't disengage from a melee at will.
Take a Power Fist only for charge protection, but you should really try to keep them out of melee.
Big D ; What am I doing wrong?
Littleboyblues ; Playing Chaos.