well see here, the thing with predator is that it's meant for junlgers that usually fall under one of two categories, ones that would like to go domination but have no good keystone within that tree or junglers that dont have early gank pressure that want gank pressure/junglers that want to take both flash or ghost. now the problem with the keystone isn't that its not useful or that it has too long of a cooldown, it is that most people tend to gravitate towards damage creep over mobility creep in case A and in case B those junglers would just take ghost over flash and take another keystone instead.
with me myself as well as a few other players I've talked to, it seems that this keystone is best on champions that want to have good map presence and constant gank pressure, but all of these champions have better alternatives that they would all rather just take. The two big junlgers that would benefit the most from predator would be nunu and ivern and in general alot of support junglers that want to have lots of presence, this among the rest of the tree is excellent for them, going predator for early gank pressure as well as high map presence later on into cheap-shot because they can all proc it and benefit from it immensely as they have low damage but consistent soft/hard CC, zombie ward so that the high amount of warding these champions can do will last even longer and give way more vision as these wards dont count towards the limit as well as adding enemy wards to your vision, and finally taking ingenious hunter which they can stack up extremely fast with assists and their massive map presence as well as the fact that these support junglers buy a large number of active items, getting massive benefit out of this rune alone.
for a secondary tree I usually recommend resolve as these support junglers also benefit a great amount from revitalize, nunu getting way more sustain from his Q and smite, ivern getting more HP back from smiting as well as giving way stronger shields to his allies, and from there the support jungler can take iron skin for healthier clears early on to be able to gank earlier and more safely at that point (remember that the bonus armor is empowered when you have a potion ticking or when you use a healing ability like nunu's Q) (for ivern its better that he takes condition instead because of his clear not benefiting from armor)
predator is generally more beneficial to support junglers in general or to junglers who would like to take both ghost and flash, but i mainly talked about support junglers here which fall under the first category.
TL;DR
Predator is perfectly fine the way it is, it's just that people dont realise what champions really benefit from this as everyone only thinks about the 'killing' part of the domination tree and never really the 'tracking' part of this tree
