We have a situation where we are required to build in an AWS region that only has two AZs (ca-central-1). In addition, there are no other AWS regions in the country (Canada). And due to our requirement around data sovereignty, we can't host this data set within a different country.
Because of this, we can't guarantee our cluster will stay up if we lose a single AZ unless we put an arbiter in a 2nd region (since no data is copied to the arbiter). However, Atlas does not support arbiters currently. I know arbiters aren't ideal, but in this edge case, it would benefit us. With our non-Atlas MongoDB cluster we can support this model, but with Atlas, we cannot. And this may keep us from moving this particular cluster into Atlas until it is supported.
Let us know if you have any questions.
Thank you!