Apple Watch on the inside of wrist?

It will work for everything EXCEPT what it uses the sensors for which include your heart rate and the ECG feature. If you read Apple's instructions on wearing the watch here: Wearing your Apple Watch - Apple Support it states the following:

"In addition, the sensors will work only if you wear your Apple Watch on the top of your wrist."

By the way, I've had the Watch-4 for a month now. The heart rate measurement and the Afib ECGs match the results that I obtain with an external gadget (Kardia) that my doctor recommended. I haven't yet had a chance to speak with him about the Apple Watch.

I hope not because that's how I wear mine. Like you, I've always worn my watch on the inside. In my case, I started 65-70 years ago mimicking my older brother and then it turned into habit.

As far as I am aware the ECG works only in the U.S.A. at this time.

The sensors work fine for me and the results match what my heart doctor sees.