And then, your projects start to scale hand your bill goes up exponentially. Firebase start to become 70% of your total business cost. You look for alternatives but soon realise that it would be an immense effort to move away since you have everything interconnected in Firebase. So you take it. Until one day, something changes: a policy, a feature, maybe your gmail account gets banned and your firebase closed. And you still have all the eggs in the same basket and can't move them.
Now I wrote this as an answer to you, but it's in reality just speculation and the reason why I avoid Google (and Amazon) services like the plague. They are competitive but just the possibility of the above scenario happening would keep me up at night.
This isn't necessarily true. I helped build a business on top of AppEngine. We did $80m gross in our first year. Our bill went up to about $500 a month. Why so low? Because we designed the system to scale from the start. We became experts in how to make the system work for us. We knew what would end up costing us money, so we engineered it from the start so it wouldn't cost us money.
Here is the case study written after I left the company...
Now I wrote this as an answer to you, but it's in reality just speculation and the reason why I avoid Google (and Amazon) services like the plague. They are competitive but just the possibility of the above scenario happening would keep me up at night.