I'd argue overly simple. The nest, and the other detectors, both have baffles in them to block out light from the outside world. However, the Nest negates some of the benefits from this by having the LED shine against a piece of plastic at an angle fairly close to the photodetector. If that piece of plastic happens to be especially reflective, or if the photodiode and/or LED happen to be misaligned, it would be more likely to trigger false alarms. The baffles in the other detectors, on the other hand, are also used to ensure that the light from the LED gets nowhere near the photodiode unless light's being scattered by smoke. The Nest design works great in the lab, but when assembled en mass, hits a bunch of tolerance issues that leads to a more jittery product.