There are a number of health issues that have been associated with spaying/neutering dogs including obesity, diabetes, cancer, cognitive impairment (Alzheimers type diseases) and joint problems like hip dysplasia, torn acls etc:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4096726/

http://www.naiaonline.org/pdfs/LongTermHealthEffectsofSpayNe...

Male dogs especially seem to suffer significant long term health effects from being neutered. A much more humane option would be performing vasectomies and leaving female dogs their ovaries.