As of today, Steinberg (Cubase, Nuendo, Wavelab), East West (Play), Avid (Pro Tools), Slate Digital, Gobbler and others are advising customers to stay put on Yosemite.
Arturia are amongst those and explain that “this update may affect operations of these instruments when in Audio Unit or standalone mode. Indeed some of them could need a release either from Arturia or Apple.” Native Instruments explain that validation of all of their Audio Units fail in Logic Pro X under OS X 10.11.
Reassuringly, “the root cause of this issue has been identified and a workaround is still being developed in close contact with Apple. The i D14 and i D22 offer WIndows 10 and El Capitan support. However, if you’re an Elektron Overbridge user (hardware with software control) you’ll come across problems.[They] will keep you updated on any developments regarding this issue.” NOTE: Mattias Witt on the NI User Forums has shared a hack to get around AU Validation. If you use plug-ins by any of the above, and let’s face it, NI Komplete, Arturia, Steinberg, East West, Avid & Elektron’s Overbridge—that includes a large number of producers—then don’t update until fixes get released. Focusrite have given the green light to their customers for their audio interfaces (but not their Midnight Scarlett plugins)… All their interfaces are compatible except the Duet Firewire and Emsemble Firewire. Apparently the Universal Audio drivers just won’t work and so their hardware won’t either until we get updates.