We had a really good time staying at the tiny home in Piha, I would give it a 8/10.
The Mercer Bay walking track which is about a 10min walk down the road is an amazing walk along the cliff tops.
When we stayed there in late June the fire place was broken, so make sure you pack warm clothes.
I personally didn't mind the eco toilet, but it may not be for everyone as your are literally doing your business in a bucket lined with a bag, then have to empty it after a day or do.
Would still recommend if you'reconsidering booking.