Don't know any pros except in situations where the animal cannot be followed up by a vet since it's a surgical procedures.
Cons, physical castration is 100% effective and not excessively invasive.
I know 2 horses who've had that chemical castration and I can follow the different changes. One of them didn't have any change at all. The other one have a bit, at least on 3 months, but that's it. I agree with Dereck, that's not worth it!
I have a friend of mine who had had a horse who had a chemical castration and his behavior didn't changed at all, and if I'm not wrong I think it's now forbidden to horses who're doing competitions! There're dangerous substances, so I think that's pretty expansive, it's cheaper to have a classical castration. If your horse has a behavior which doesn't make you in danger, I don't think it's worth :)