I was just introduced to Firestore (in Datastore mode) last week at work. (Enterprise dev.) Should I be afraid? I've heard so much shit about Mongo that at least I hope we are not dealing with as bad a product as it.
If you can design your schema and usage patterns so as not to hit those limits, it's a very scalable option. But in return, you are in for a very strong vendor lockin.
Unfortunately, almost all pay as you go, linearly scalable managed databases, for example, FireStore, DynamoDB, CosmosDB are essentially lockin in nature (and maybe by design)!
Other OSS NoSQL products almost always need a base cluster costing $1000 (or more) per month to get decent baseline performance. After that point, you can scale to a very high scale by vertically and horizontally scaling your cluster.