Yea its the downfall of the o-ring seal.....lol no seriously it wont leak if the o-ring is in good shape and you dont try dry tightening or overtightening it.
I have 3 two little fishies reactors. I have never had any of them leak on me. I agree you shouldnt run carbon and gfo in the same reactor, especially if you want to get full use out of both mediums. The brs onez work well, they work on the same upflow principal...but they can be messy when doing maintenance. if you dont have room above your sump to mount the brs reactors, I recomend going with something else. Two little fishes being the next most economical imo.