Compared to PostGIS SimpleGeo is seriously lacking. You can only do the simplest of queries, and it has tiny limits on most accounts.

The problem is that spatial data tends to create strange usage patterns compared to other software. I run PostGIS installs with tens of millions of features, most of which will never even be queried. Different spatial queries can result in dramatically different resource needs. I need a service that is affordable to store multi GB datasets, and can query them very quickly. SimpleGeo have not come close to solving this.