Eiter can have benefits and disadvantages if built right and are strong but can still flex.
As stated with this post
i have a 32 spitzer altered i have ran both with the duel bars you have to make sure the height on both wheels are the same if not the one that hits first will drive the car while it is off the ground. with a single dead in the center car will go much straighter. thats my experience my car has 1.1160ft off the ground 5.10 1/8tm mile
That is partially correct. A double roller wheelie bar setup can definetly steer you. In most High Horsepowered cars You Do Not Want the rollers to be an even distance from the ground. Usually you set up the left side to be a little higher becuase if it is the same ht from the ground it will usually hit first and will unload the left rear tire enough to move you to the left.
IF your car is slamming the wheelie bars down you need suspension tuning, no matter what kind of car it is. Wheelie bars can be a suspension tuning aid if you are touching on them as you should be. They are definetly a safety device and should be on any car that will pick them up.
The single Mono Wheel if built right will not steer the car and when at rest can be moved slightly side to side for some flexibility. They do work very well in a well behaved car.
If the car is unrully and crashing wheelie bars hard to the track, there is no wheelie bar that will not fold with enough force on it.
I have a customer who has crashed and trashed three sets of wheelie bars of both kinds last year. The last wheelie bars they trashed the car had all 4 tires in the air and the broken wheelie bars was the only thing touching the track. I wish someone had taken a pic or vid of that. I was told by several who were watching it. They don't just trash wheelie bars, they get violent tire shake and break expensive parts like racing trannies etc. The heck of it all is I know I can make the car work. The guys in the chassis shops who have tweekd on it, can't.
The longer bars are more forgiving but you need to have the rollers higher off the track. I think I still prefer the two rollers.
Ed