How powerful is Planar Binding?

The Planar Binding spell is the answer to the classic trope of dark mages summoning and binding demons to their service. But what is the extent of this spell’s capacities in 5e?

1. What scope of orders can be given? Can suicidal orders be given? Even further, could you order the being to repeatedly attack itself until death (the closest thing to suicide in 5e)? Can you order a bound creature to be a “willing” participant in a spell? Spells like Plane Shift and True Polymorph depend on the willingness of the target to avoid saves.

2. Can truly any fey, celestial, fiend, elemental be bound? MToF added stats for demon princes and a lord of hell, could such fiends (assuming you could beat the resistances) be bound?

3. Does a creature need to understand your orders to obey? This might seem like a dumb question, but certain beings might have too low intelligence to understand language, what kind of orders can they be given?