Do you have a sense of what that limit is?
There's a pretty big price difference between Spanner and Aurora at the entry level so it's useful to explore this.
Per their pricing page it looks like the largest instance available is a "db.r3.8xlarge", which is a special naming of the "r3.8xlarge" instance type which is 32 cpus and 244gb of memory.
That's a hell of a lot of capacity to exhaust, especially if you're using read replicas to reduce it to only/mostly write workloads. Obviously it's possible to use more than this, but the "sheer scale" argument is a bit of a flat one.
Let me present a quote to you:
512 kb ram ought to be enough for everybody
I don't have a good idea what the upper limit is for an Aurora database setup.
See here for more info: https://aws.amazon.com/rds/aurora/faqs/#high-availability-an...
For the former, I don't think they specify beyond "automatic".
For the latter, "service is typically restored in less than 120 seconds, and often less than 60 seconds": http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Aurora...