The Nissans that I have encountered were built by Tohatsu. Generally from what I have read they are excellent engines. I had one of their 8HP X-LONGs for a sailboat and mine sure ran great.
Don’t know about 9.9s but the smaller motors seem to be engineered well. However, they are NOT good at ALL in salt water, as the cooling systems plug up something fierce. The aluminum they use is horrible in salt.