When I bought my B6100, it had the exhaust turned upright. I used it like this for 2 years, till just a month ago I turned it back to it's original horizontal position, before I catch it in some overhead tree branches and break the manifold off, as I use it for a lot of mowing in the summer. Sometimes I have to duck under the overhanging branches, and more than once I had quite a bit of pressure on the end of the pipe. Remember you want it high enough so you don't eat exhaust all the time.
Besides that disadvantage, the only other obvious one to me is if the machine is left outside all the time and there isn't a cover over the exhaust end, whether it be a flapper or a tin can, then water will get in there, and if the manifold is like mine, which has no water drain in it, the water could end up in a cylinder. I always store mine in the garage so this wasn't a problem.