One partition of DDB is incredibly tiny compared to one partition of an RDBMS. You can push that one partition of RDBMS pretty far before you're forced to design sharding into your system. With DDB you are basically forced to design sharding into your partition keys up front or you will have hot partition issues. This is by far the most common problem I see with teams using DDB, so brushing it off as "with good design to avoid hot partitions" is understating the scope of the problem.