Any dewormer can cause colic if administered to a horse with a large worm load. It isnt the fault of the dewormer since its just doing its job. Some do the job a good deal better than others and if the horse has a large worm load, it can cuase a colic which is why this must be managed carefully on neglected horses or horses with an unknown deworming history. Zimectrin is just ivermectrin. Zimectrin gold is just ivermectin with Praziquantel so it kills tapeworms also. I don't have a problem with those ingrediants.
However, all that said, I do not use Zimectrin at all due to the large reports of chemical burns/adverse reactions of the mouth being reported by users all over the internet. Apparently the carrier agent can cause chemical like burns in some animals. I don't feel the need to take the risk personally. If you use it and your horses are fine...then its no problem right? If they have a reaction, you will not be so happy. Chances are itll be fine...but then again they may not be..I feel its a risk everytime you use it so I choose not to. zimectrin dewormer causing mouth burns - Google Search
On another note, I use Quest Plus now that its back. All the horse owning vets I know use Quest Plus almost exclusively. I think that speaks volumes. Used correctly, it is the most effective dewormer on the market currently.