In regard to ChrisA scenario, It's good to be paranoid and assume that 2 things might go wrong IMHO.
Let me give a tangentially-related example. A buddy and I were cavern-diving, and the reel got jammed on the way out. When things get a bit high on task-loading, are on your way out and eying the SPG and dealing with flow and a jammed reel, some additional mistake can easily happen. In his case it was bumping his buckle and dropping the weight belt, so now he's pinned against the ceiling. Now we've got 2 problems and it took a worrying long time to untangle things. See what I mean?
If you predicate your assumptions on I always do low-current 30-foot dives and a single failure is not the end of the world.... well you are leaning to an attitude that says a thin safety margin with best equipment is okay. Just think about it when building scenarios. What if your buddy-breathing off a 2nd in some nice safe pool? Everything is fine right? But add in the 2nd problem that your buddy has a good shot of adrenaline and breathing hard. If that reg is low-performance you have a PROBLEM. So make sure if you are shopping one of these widgets that it's got good performance. I am not just picking on integrated people here, I've seen plenty of people with backup 2nds that are these tiny things with poor performance that I think are a bad idea.
One obvious one I could see happening is you get distracted with the whole situation at hand, you are ascending and breathing off your integrated reg. But as ChrisA points out you may have to take the thing out of your mouth to vent the BCD if you don't a pull dump on the shoulder. Do you do this smoothly? Do you remember to blow bubbles without a regulator in your mouth to avoid lung expansion injury? People generally train for OW cert with a "standard" setup if you switch to integrated you'd better think about how this could change your failure scenarios and procedures in OOA emergencies, not just add it and enjoy the benefits.
The first diving injury I witnessed in 97 was an under-weighted diver who was kicking down at the safety stop. It took me a minute to realize why he was head-down and kicking and it was too late to stop what happened next. His underweighting inconvenience became a diving injury when while dealing with his additional task-loading he forgot to kick for a moment while doing something and thus bobbed to the surface, and also held his breath. This led to a minor lung expansion injury, subcutaneous emphysema technically, certainly not the end of the world but my first example of how one problem can quickly lead to a 2nd one. He only had the "rice krispies" under the skin of the neck, sit out diving, and a hospital checkout.
I don't go much further with my paranoia than most people, I mean it's not like I'm diving Y-valve and double-regs on a 60-foot dive or anything. But I try to play out all the what-if's I can and account for them.