Generally, Quad Darts is used as an early-game upgrade when using the Base Heli Pilot. Similarly to the lower tier, the Base Heli Pilot finds most use on long tracks and maps where the infinite range is appreciated.
For MOAB Shove, it may be better to not buy Quad Darts so that it can stall blimps for longer, but for Comanche Defense and Comanche Commander this crosspath will be the best as they are DPS-oriented and can obtain camo detection from external means.
Quad Darts has not been changed since it was first released, but it's now more expensive compared to its BTD5 counterpart.
Quad Darts costs more ($600 → $800).
Heli Pilot manages on pursuit far too easily without any need to consider Bigger Jets unless camo or downdraft is required. For more crosspath variety than just 'camo' we moved more of the base Heli speed out and back into Bigger Jets with an overall buff to Bigger Jets. This should allow Bigger Jets to scale a little better than other helis through early speed ramping. [...]
~ Ninja Kiwi, referring to base Heli flight speed versus Bigger Jets
Affected by base Heli flight speed nerf versus Bigger Jets crosspathing
1-0-1 Heli appearance
Crosspathing in BTD6 (Top: Path 2, Bottom: Path 3)
When the entire list of BTD6 tower upgrades are listed alphabetically, the Pursuit upgrade comes right before Quad Darts.
Quad Darts is the only returning Heli Pilot upgrade below Tier 3 to be more expensive on initial release.