Yes, a few operations with GEGL have been failing me, too.
However, GIMP already can use GEGL for compositing its layers using GEGL and do its color manipulation using GEGL ops, it however still stores buffers in TileManagers and the integration with GEGL is done through proxy GEGL ops that fetch data from a TileManager or store data back in a TileManager.
According to the roadmap here
, GEGL will be fully intergrated with GIMP in the next version of GIMP 2.10.
Also, you might be interested in reading the article here
about the progress made to port GIMP to GEGL.