By slab, are you talking about a log slab, or a flat board type slab ?? How thick a wood ??
I watched a guy in a craft show, one time, that used a hole saw and an expandable wood auger bit, to make the mortise and tenon cuts.
For "Rustic", log slab types, use limbs and cut the tenon using a deep hole saw. Center it by eye and cut to the depth of the hole saw. Then, carefully take away the outer parts leaving a very slight tapered shoulder. Bore the Mortises about 90° to the curvature of the log slab. Make them just a few ° splayed out from exactly vertical at the ends of the slab, and that makes a nice Rustic Bench.
If you are using "Flat Stock, cut the legs out and turn the Tenons on a Lathe or use the hole saw idea. Bore the Mortises with that same Wood Auger Bit, being careful to not go through to the finish side of the flat slab. Once the holes are bored, you have to line up the legs for the "Nice angles" and mark the end cuts so they drive in and seat properly in the bottom of the holes. For square legs, be SURE to make them so the legs are all lined up the same, so they don't look hap hazardly driven in.
These are not difficult to build, either design you choose.
Finish might be a problem, so, something similar to Deck waterproofing might be OK ?? If you can seal the wood effectively, every spring, just sand down and put on another top coat of sealer ??