Yes, garlic certainly is effective, it saved my life when I had a bad roundworm infection, along with clove and tumeric. Passed hundreds of fully intact worms. It just takes a while and a good amount of it, most people can't stick it out due to the smell.
It can constipate you also temporarily, as will anything that kills worms.
Regarding whether or not it destroys the good bacteria, I guess it depends on what you read and research. For me, without replenishing the bacteria, I was great. I have read that it acts as a prebiotic, but who really knows what is true regarding the gut and parasites, we can only go by our own experiences, as we are the true researchers in this mess.