That's a good question!
But I don't know, its all trade offs.
Gearboxes are probably going to be more robust, possibly handle more torque, and have bigger motors. They'll need seperate escs, take up more space, little more wiring involved. Shaft to hub adapters aren't always available how you need
Gearmotors are mostly sealed, need escs, more mid-size motors. Lots of ratios to choose from, usually have bearing or bushings available.
Servos are pretty well sealed, more self contained, driver board can be used as esc, and often cheaper to start with. Hub adapter is easy with just a splined servo horn. 2a stall is around the most you'll see without letting out smoke of your driver or rx.
Mostly my decision for this project was based around: i still had a 4 pack of these servos already here for a cable crane that i hadn't really started converting. Instead that crane will probably get something more like the hs645 like i run in my Michigan T24 crane
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Now that it's too late for dads, I also ran across this Dozer conversion that looks really slick. Their drive servos fit better than what i used, and the dual-servo blade lift looks slick using the rams
https://youtu.be/zEvEBDnsqXc