Oura was quite good with warning me that something is not ok and I should take it easy on the morning of the day when I started to have symptoms. I was tested positive later that day. Interestingly, nobody I met days before I was warned was infected, so in my case warning came in right moment.
I was going to mention them. Some members of the US military were given them as part of a study on COVID detection. Basically any wearable sensors covering the right metrics can be used for this. The question, really, was less about "Can wearable health monitoring equipment detect early signs of <disease>?" than "What are the early signs which wearables can detect and use for prediction once we know them?"