The WakeMate wakes you similarly to how you would wake if you did not set an alarm. That is why it is very unlikely to have adverse effects.
Regarding your comment: "I think the original commenter gets that you're just waking up a little earlier, but consistently doing that every day isn't natural." It may just be because I'm familiar with the product but it doesn't seem to me that setting your alarm for 9:00am and waking at 8:55am can be construed as unnatural.
What is the size of the effect in the research?
"1. The optimal wake feature reduces daytime grogginess only in the sense that it puts you in a better mood in the morning. 2. Yes it is very unlikely to have adverse effects. You are just waking a little bit earlier at a point where your body feels more inclined to wake up."
Two claims. No data from either published literature (from other studies) or their own data. How do they even know it works? Why should we treat it any differently than claims of magnets improving your sleep? A free pass shouldn't be granted just because they are YC.
There are probably legal issues with your claim as well. I'd just say that it's very unlikely to have adverse effects instead of "it is completely free of adverse affects".