I've done Depo Provera shots on a mare in the past. About once every 45 days, and eventually as she got older, I didn't need them anymore. We speculated that she just really had painful ovulations. My vet (at that time, 7 years or more ago) advised against the implants as they left considerable scarring at the insertion site and had to be redone every 45-60 days. The marble was a new thing then and not many did them, so he didn't have an opinion on that.
With Regumate, it isn't just that it's a hormone that can screw up our (women's) cycles and hormones, it's a known carcinogen. I always have used gloves & a liquid proof apron when handling that stuff, it's just wicked.