You can check this spreadsheet I made. The Black Overo can be rarely found wild among other mustangs, being ridden by Tier 3 lawmen (Marshals and Pinkertons) and hitched near where the Mustang Cigarette card is, at Downes Ranch (about a ~5% spawn chance).
Everything edited has to do with horses. Vanilla Pinkertons are aimbots, which is already broken as it is. In the mod, some of the rarest horses can only be found being ridden by Pinkertons. So if you want to attempt getting one of those for yourself, would you like getting oneshot every single time? That would make it very hard to get the horse, right?
Wanted levels have been modified because, again, some horses can only be found by lawmen dispatched only at certain tiers, and in vanilla from wanted level 4 to level 5 there's a gap of $750 which is too much. Staying alive fighting lawmen from a $250 bounty to a $1000, and then after all of this attempt to steal a horse while on a $1000+ bounty while getting spam-shot would be very hard, right?
The mod doesn't "spawn more people". Not even more horses (except on specific town locations, only 1 per, the rest in the wild). Just different breeds. Ped pools haven't been touched in any shape or form.
"So many things get added to towns". Additions per se, the mod literally only adds 6 props, of which 3 are in towns. A single hitchingpost in Armadillo, Rhodes and St. Denis respectively. Armadillo is already pretty much a ghost town. No other props have been added that would make you "uninstall the mod right away".