Oracle OEM is slower, more complicated and it if it breaks, which has plenty, it’s super hard to fix.
OEM has nothing like visual sql tuning diagrams. If OEM can’t tune a query they you are at a dead end street with no way out. With DBO you’ve got the VST diagrams which explain tons about what is going on with the query
There is no load testing or SQL IDE in OEM
OEM’s SQL Tuining Advisor (STA) has a solid history of not being able to tune queries or worse still “tuning” the query and making it run slower