Are switch screws sufficient for load grounds?
At switches

Grounds at switches must be pigtailed to the box, then all the grounds from all the cables joined to the box with a wire nut.  

If you don't want to use wire nuts, you could a) use box clips on the edge of the box, b) drill and tap any #10-32 holes in the back of the box for each ground wire and land it on the screw.  

You cannot land one wire on the box and one wire on the switch.  This defeats the entire purpose of pigtailing grounds; ground paths should remain continuous even if you remove a device. 

At luminaires

It depends on how the luminaire is built, and you have to make a case-by-case judgment call.  

For instance some luminaires have two ground wires -- one for the ceiling rose, and another one for the lamp proper which is on the end of a chain or cable.  Chains are not valid grounding paths!  In that case, you can ground the rose via the metal box, but must wire the ground that runs down the chain.  

If the luminaire has some plastic and some metal parts, that problem may recur.