Doesn't seem reliable.
If Netlify goes down, you won't be able to publish, but your statically rendered site should still load just fine unless you are using it to make dynamic queries on the fly (which is, I think not generally done for Gatsby, but very possible to do)
- My ISP
- My power company
edit: Also they're third party. 1st is you 2nd party is the customer (The blog audience).