So like I said, this is my first attempt to title a working dog and whatever information I throw out there is only my personal opinion, keep in mind that this is not to be taken as fact.
Is the reason for not doing both that it would be too hard on the dog... or too confusing for the dog?
I personally don't think so, at least not with my dog. When you say this, I'm interpreting this as biting/ targeting. My dog knows when the sleeve is in the picture on a helper, we bite the sleeve. When the suit is in the picture with the helper, we aim to target the lower legs. Doesn't mean if you have a sleeve on and throw her a kick, she wont tag your bare leg, it just means she tends to zero-in on what the target-object is. I can't say i've done a whole lot of reinforcement with this, because it just comes naturally for this particular dog. I will say that i've seen a really nice malinois started in ringsport only to later be transitioned to IPO and his entry on a long bite was low and center-mass (which was obviously extremely dangerous for any schutzhund helper on a hard-hitting, fast dog). His handler spent a lot of time trying to undo his default targeting and approach by trying to get him to target the sleeve appropriately. This is something that will likely always need to be addressed for safety concerns.
I have to admit, I'm blown away watching vids of dogs doing French Ring. (Not that I'm unimpressed with IPO) I'd like to get his first IPO title before adding in another sport, though.
I also find ringsport to be a much more impressive sport to succeed in or even watch, but thats because I believe FR is a sport that tests the raw material of the dog, whereas IPO is a sport that tests the training of the handler. I think that as a first time working dog handler, you will learn a lot more foundation in good-training techniques from people with an IPO background because the exercises for that sport are so intricate and piece-y. You learn to break down an exercise into different parts that allow you to ultimately build your dog back up through repetition/ pattern training into showing off the cookie-cutter picture we all strive to see on the field. Ringsport, on the other hand, I feel allows you to use your variety of training skills to showcase a dog's natural style and ability for completing a task in an effective way.
I personally recommend doing IPO as a primary, because you'll get a lot more input on how to become a better trainer by learning a strong foundation in technique. I also feel that ringsport, being dominated by the dutch/ mal people, tend to have different levels of expectations from their dogs at different time frames than GSD people. I've had my fair share of setbacks in trying to train my girl like a malinios when she is in fact a german shepherd. I will confidently say that they do NOT work the same and you cannot treat them the same when training, expecting similar results... But, I learn as I go and I grow from it. My approach is all about the experience, not the end result. I've learned more by making mistakes and fixing them than by succeeding at some kind of slow, structured pace. My dog is forgiving and I think the experiences we've had in the past two years that we've worked together has made us a stronger, more well rounded team.