So, obvious ones.
Light linking : I doubt that has an impact on speed of development to mention it. But we cannot say that is not easy with BI and complicated to use a compositing set-up with EEVEE and Cycles.
Baking : Cycles process for baking with selection of a target Image node that have to be in material although it should not be contribute to it. It is awful to set-up. Awful to bake several textures.
Particles Rendering : With BI, by default, particles are rendered as halos. With Cycles, you have to set-up a domain with a volume shader and use a point-density texture to obtain something close or again, you have to use compositing to use node to blur or boost a render of particles rendered as sphere instances.
EEVEE have to follow same process.
Currently, when a particle system is added to an object, user is seeing dots in Viewport. But if he makes a render as is, he obtains nothing. Default Halo display type does not correspond to anything, now.
Freestyle Pass : For BI, under Renderlayer Tab, inside Layer panel, there is a Freestyle Include option. That allows to disable render of anything, except freestyle lines. That is missing under Cycles UI.
And now in 2.8, it is missing for EEVEE, too.
We may need to composite several freestyle lines passes. If we can not decouple rendering of lines from solid meshes rendering, that is really annoying.