I have run into a couple of challenges doing both agility and obedience, but they are mostly amusing and not serious. My AmStaff, for instance, sometimes will shriek when going from front to finish left. He loves the swing and gets pretty excited. He has never done this in the ring, but will frequently do it in warm ups. It always gets me dirty looks! Also, on a recall, he comes in like a bullet. It's scary to stand there and trust him to dump off speed to get stopped before crashing into me. (He never has hit me.) And when heeling in fast time, he sometimes hops a lot out of excitement. While these things are not typical in obedience and some trainers consider them problems, other people tell me how much fun it is to watch him work. In novice and grad novice, he has never scored lower than a 193, so again, not serious. When I toss the dumb bell, his tongue literally curls and his lower jaw bouces and he trembles. Looks pretty nuts. But he always gets the dumb bell! This juice is likely fatal to any OTCH dreams, but it shouldn't prevent me from earning titles or even wins in the A ring especially.
In agility, I have to be careful about eye contact and saying his name. He will drop bars if I ask for attention when he's over a jump. I had to teach him not to stare at me on course. Again, a challenge, but one that was trainable, lots of targets and forward tossed rewards. My obedience work has given me very nice start lines and control. I have never had a "Zoomie" dog and my amstaff can focus like crazy. We once had a rabbit run through a ring at an indoor trial while we were competing and my dog kept working. My amstaff is a very fast dog. He has broken a weave pole and taken down the tire jump and has barrel-rolled off of the dog walk. This isn't a trotter. I needed to find a good coach to help me with handling skills because all of my dog's ugly moments were the result of unclear handling and my failing to cue collection. That failure on my part was dangerous to my dog. I am growing as a handler and having fewer heart-stopping moments. (and less massage therapy and chiropractic work on my poor dog!)
I think that if I really wanted to be the best, I would need to chose. But not to just be competitive. And I probably lack the discipline to be really competitive anyway, so what's the harm in cross training. I love it, my dogs love it, and no one else is impacted by my choice.
In rally, I expected obedience heeling. I also handled nearly silently. Still, I think rally really screws up good heeling by adding so many doodles. However, good heeling really amps up a rally score! We did very well in it. My dog moves SO fast that we often won our excellent B classes. That was always a little baffling. All we had to do was hit the 100 and we always won on time. Sometimes I felt a little bad about that because there would be dogs in there who were more technically correct and better obedience dogs, but they weren't as fast. In rally, they don't take away 1/2 point deductions, so a fast dog who was close to correct could beat a dog who was prefect but slower. But thats rally. When I think about an RAE, part of me is interested in trying because the wins are sort of fun, especially with an off-breed like an amstaff. And now some trials are giving out High Combined awards for rally. I love those gorgeous ribbons and suspect that the only ring that I will be able to earn them in is the rally ring. We are both too random and lack the attention to detail to get them in the obedience ring! And that's not likely to change.
My amstaff is my first performance dog. Before him, I hadn't ever been in a show ring. I had 5 CGCs and 2 TDI therapy dogs, but that's different. I am learning as I go and having a lot of fun. Any failure's are mine. My dog is honest and biddible and joyful. There is so much to learn and I am so new to all of it. My dog is utterly forgiving and totally driven. The drive can be over the top, but it is also a great tool. I can drill something new 50 times in a row and he's still happy to do it. Flip side, he can lose his mind with over-excitement. It's been a journey and there's a whole lot of road ahead.