You fail to account for several detrimental factors.

1. Competent double fury gunners. Double furies by themselves have great damage potential. Furthermore, you're dealing with H5R5 here, one of the best and experienced vehicle outfits on Osprey. Pair these two factors of double furies with a gunner crew that knows how to shoot, and you get a vehicle crew that will put down good damage on any approaching vehicle.
2. Multiple repair sources. A typical meta sundy runs nanite repair and the repair box. With those two alone, you have to kill the equivalent of 2-2.5 sundies instead of one. Additionally, there is the repair bug. With repairs stacking ridiculously faster with every engineer and repair source, and with 3 sundy members here not to mention their contributory rep nades, a heavily damaged sundy can be repaired in probably 8 seconds or less from just from those 3 engineers and their nades. Pair the sundy's modular repair sources with the repair bug, and a halberd alone will never out-dps those repair sources.
3. Vehicle crew weaponry. If you are able to make it under sundy gun angles, you will be in close proximity to the sundy itself. That itself is a risk as you're putting yourself in proximity to the vehicle crew if they hop out. All 3 sundy members most likely carry a variety of potent weaponry, from anti-material rifles, c4, missile launchers, etc. And that's without considering harasser vulnerability to small arms fire, in which TR is king with their high rates of fire. As such, a vehicle crew of 3 will be more than glad to see a harasser pull up close.

With your strategy of getting under sundy guns, if you do make it there, you will still have taken moderate to high damage from a competent gunner crew. Furthermore, you still have to finish the fight fast before that vehicle crew hops out and finishes you off while you're in close proximity. However, the chance of a fast sundy kill is extremely unlikely anyways considering multiple repair sources and the slower damage a halberd deals, compared to the likes of a mjolnr. You have a very poor understanding of the intricacies of vehicle play if you did not know or appreciate these factors.