...are herds of sheep actually effective at their job, assuming an appropriate adjustment in conditioning expectations?
Flocks of sheep can be very effective. Inexpensive and flexible too.
Mowing by sheep and grazing animals was pretty standard at GB&I courses for centuries/decades. Some of us wish they still were. See my other old photo threads, like the ones on TOC and St Enodoc and you’ll see that in prior times sheep were nibbling away at the fairways. Other parts of the world too. Not appropriate everywhere though for reasons like climate and nearby hungry meat eating critters. Goats are good nibblers too, eg Lahinch, and will eat a greater variety of vegetation than sheep or cattle.
Conditioning isn’t usually an issue unless you’re a course-must-always-be-manicured fan, and for rural area courses there is a cost impact in terms of costly equipment and staff versus grazing animals. Also, a course doesn’t need that many sheep, they’re pretty much 24-hour nibblers and the resulting short length of grass is pretty near perfect to play from and the sword of high quality. Sheep tend to avoid putting surfaces as the grass on the fairways is easier for them to nibble. Bigger grazers with heavier feet can need a stout or electric fence to keep them away from putting surfaces, eg Brora.
Atb