From a rigging perspective: For me, the big essential feature for rigging is the new Shape Editor for blendshapes (introduced in 2016.5) that includes delta smoothing, masking and eraser brushes. It makes a blendshape workflow 20x more powerful than before. Everything else (for my workflow) is just menu-shuffling, less contrast on the icons every year, and a new variation of crashes to contend with.
And it is easy enough to export the blendshapes back down to version 2016. However I found that I had to duplicate off the shapes so the shape masks don't bake down as weight maps in older versions of Maya. That was causing weird issues when exporting to game engine.
From an animation perspective, there were a lot of weird bugs with the new graph editor and the "classic Coke" graph editor, depending on which 2017 SP we were on. Infinity wouldn't work. Or it wouldn't display properly. Insert keyframe and selecting curves has changed a lot (for better or worse, I don't know, but our animators were griping).
I haven't tested 2018 yet. But I'd do at least a few basic workflow tests before committing your team.