It absolutely is from what I've heard. There are devices where the Mali GPU works well in some games/apps with one version of the OS and are severely broken in a newer version because the driver broke a lot of things to make other apps work.
It's one of the big reasons I'm hoping that with efforts like the driver for GPU for the RPI being open sourced that we'll start to see some more consistent drivers for mobile chipsets.
Or move the whole rendering to a server in the cloud, where any awfulness is at least going to be consistent and predictable. This is what NV were (are?) pushing as "Grid", though I haven't heard all that much about it. They say they can keep latency at acceptable levels, but it strains credulity a little bit.
It's one of the big reasons I'm hoping that with efforts like the driver for GPU for the RPI being open sourced that we'll start to see some more consistent drivers for mobile chipsets.