Apple made some pretty significant changes in Mac OS X Yosemite and El Capitan to the way the OS boots and functions. New hardware that was sold with Mac OS 10.10 Yosemite’s release benefited the most from the new OS in the form of optimized drivers. However drivers used to support the older hardware have not faired as well and complaints of poor and/or buggy performance have been common.
As a result, we continue NOT to recommend anyone update to El Capitan if their Apple hardware shipped originally with Mac OS version 10.9 or less. Both 10.10 Yosemite and 10.11 El Capitan performance on older hardware that originally ran 10.9 or lower have been below user expectations, resulting in client dissatisfaction compared to the experience with their previously installed Mac OS version 10.9 or earlier.
There have also been recurring residual problems from users who have updated from older hardware to Yosemite or El Capitan.
http://www.macissues.com/2014/10/20/fix-os-x-yosemite-always-booting-into-safe-mode/
http://fieldguide.gizmodo.com/the-worst-bugs-in-os-x-yosemite-and-how-to-fix-them-1652690924
Though there are reasons to upgrade, there are also very good reasons not to. Each user must carefully consider the consequences of either choice.
At present, the only times we would recommend anyone update to Mac OS 10.11 El Capitan is if at least 2 out of 3 of the following conditions are met:
1) Your machine originally shipped with Yosemite (10.10) or higher
2) Your machine is experiencing problems in Yosemite
3) A critical piece of software you require to do your job specifically requires 10.11 El Capitan or higher in order to run.
I hope this helps!