Rendering is quite different in Unity 5.
As a background, I used Unity till this Fall, including Unity 5, and have switched now to Unreal Engine 4.8. I am using 3D Coat for sculpting, texture painting and Retopo / normal baking at the moment, which handles the differences between the two engines for me.
In the process I tried importing models baked for Unity 5 into UE4, and also the other way round. You will isntantly see lots of shading errors without adjustements
Normal Maps are especially standing out. They seem to follow different standarts (I guess DX vs OpenGL)... but I am not 100% sure it is only the normal map, could also be that the normals are claculated differently. Now, this could very well be a problem with the way 3D Coat exports models to FBX/OBJ, or with the import settings in both engines (which I usually leave at the standarts for UE4 with good results, but need careful tweaking in Unity 5)...
In the end following Hodgmans recommendation would be wise. Unity is MUCH more fickle when it comes to which models get rendered correctly and which exhibit all kind of weird shading errors than any other of the 3 big engines I have tested to date. It is certainly not the fire-and-forget process that is importing 3D models to UE4. Also, their PBR Rendering shaders ARE kind of different than what UE4 does. And then there are the different renderers... Forward, Deferred, Legacy deferred... each influencing how stuff looks aside from performance impacts. Whereas UE4 AFAIK just gives you deferred.
Unity seems to stick to incremental updates to their original engine from 2007(?), whereas Epic seems to rewrite the engines more or less from scratch, which might explain some of these oddities.
Get the free version of Unity 5, and test your model with the same setting as the client needs. Maybe let the client provide you with a "test setup", some kind of project that allows you to drop your model in, add it to a testscene that will then use the exact same postprocessing and camera settings as the final product to make sure your model does work with his settings.
That might not work 100% as there might be some things your client cannot share (like 3rd party assets from the asset store), but it is definitely worth talking to the client to see how you can best test your models for Unity 5 to give him a smoother expierience with your services.
EDIT: Oh, and VERY IMPORTANT:
There are thousands of different 3rd party shaders in Unity, especially the different PBR shading systems are still widely in use AFAIK. Some of these are quite different than the Unity 5 standart shader, so definitely make sure you ask your client what shader he/she uses exactly. That might make a big difference on how useful your model is to him/her