Dunno, for some reason I am able to repartition, resize and manage my disks with the new Disk Utikity just fine. There were certainly some shortcomings with the 10.11 version (window couldn't be resized etc), but weren't they fixed in sierra? What is it that you are still unable to do?
While in theory I could upgrade to Sierra by installing newer firmware I'm not seeing any point doing so at the moment because Mavericks works well for my purposes.
I would like to know why Apple chose not to fix El Capitan Disk Utility given that its the last supported OS for many of the older Macs...?
As for what doesn't work in El Capitan Disk Utility it would be easier to list what works, the whole software is half-assed and I'm fairly certain whoever wrote it was drunk on the job...
Some examples: RAID support only in CLI, unreliable partitioning (Mavericks Disk Utility has no problems with the same drives), Text cannot be copied from the logs which is really big problem for troubleshooting and Disk Utility provides very little additional information compared to Mavericks version.