Being an installation and maintenance contractor, I've seen too many jobs were the GC wanted too steep a slope in a landscape setting.
Against my advise, they opted for fine lawn on a steep slope.
5 years down the road, these things are happening:
1.) The property owner pays more because maintenance contractors charge more for cutting those steep areas.
2.) Contractors dislike the exposure of putting a mower on a steep slope. Rollovers and out-of-control machines are not uncommon. Add that to spinning blades and you've got a dangerous situation.
3.) The mowing, whether done by a mower or line trimmer, always looks like crap. Mower tires leave ruts and spin-trimming a large area always looks poor.
Moral of the story: Plant other materials, install retaining walls, use a groundcover, etc, but DO NOT install a lawn area with a slope too steep to mow quickly, effieciently and safely.