The sensitivity of Bullmastiffs is perhaps the biggest negative for me. I heard about it already but didn't appreciate how big an issue it was until I had to live with it. For such a large intimidating dog, they are real softies! I'm all for motivational training, but in the real world you do have to use corrections if you want a well behaved dog. My American Bulldog knew when I was disappointed, took it well, and moved on. My Bullmastiff sometimes acts like I'm abusing him. He's like a little baby sometimes, and no I'm not just talking about hitting him, it can just be a verbal correction. It's weird. If I'm training him and give him a mild/moderate physical correction in an angry voice he pouts and shuts down, but the other day I banged his head hard against the counter by accident and he didn't even flinch! They have a very high pain tolerance, so the emotion affects them more than the physical.