Mini Split Condenser wall mount on wood exterior wall
If the brackets are lag screwed into plywood, I wouldn't trust it.  There isn't much meat for threads to work with in 1/2" plywood, and there is going to be some vibration from the compressor.  



If it's held in with toggle bolts, 



it will be very strong, I would not be worried about it pulling out.  But I'd still rather not have a vibrating piece of equipment of this size and weight hanging on the plywood sheath.  I'd be concerned that the weight would warp or otherwise deform the plywood over time.  

To make an attachment like this, you could bridge across studs with three horizontal pieces.  You'd anchor the horizontal pieces into the studs with long lag screws, and anchor the brackets to those pieces.  

You could use 2x4's for the horizontal members, and lag screw the brackets into the 2x4's.  The 2x4 will hold the threads much stronger than the plywood.  You could paint the wood and it might look pretty good.  

I would probably use 7/8" tall strut channel for the horizontal pieces lagged into the studs:  



and strut channel nuts to attach the brackets to the channel:  



Some consider strut a little too industrial looking for home use, but I think it might look fine in this case.  It will make such a strong mount you could get by with just two horizontal pieces bridging the studs.  After all air conditioning compressors are not invisible, you are going to have to look at something here.  The strut is quite a bit smaller than a 2x4 and once it's painted I think it will look good.  

This is a slightly harder install than using 2x4's.  The vertical spacing between the two pieces must match the spacing between the holes on the brackets, and needs to be pretty precise.  The easy way to do that is attach the brackets to the strut FIRST, then screw the strut into the studs.  

The hardest part here is going to be disconnecting and reconnecting the unit.