3/16"ish short paired Y-shaped knives hanging from some sort of D-ring survive rocks the best in my experience but don't chop thick wood. Tied for worst seem to be long heavy paired L-shaped knives and long thin hammers. Hammers tend to break where the head narrows above the wide knife part. Long heavy L-shaped knives hit rocks where the elbows of the Ls meet and bend outward and upward, leaving you with curled knives and a funny uneven cut afterwards.
If I were looking for a mower whose primary use would be for mulching light cut brush with some rocks sticking up into the mix, I'd look for something with VERY heavy (5/16" thick) Y-shaped knives on a D-ring or wide, short, heavy hammers. Hammers, IMHO, do a better job on sticks. For cutting standing brush and mulching afterwards, I'd do two passes- heavy rotary capable of at least a 4" stick for knocking it down and a heavy flail for chopping. A tractor with a really slow gear is required, because high-hp tractors with break even heavy mowers if fed too quickly.
Either that or get the REAL solution for mulching sticks and trees, an FAE forestry or ag mulcher.
Fae Usa Pasture mowers These're for people with money/horsepower to burn or who have enough work to do that they can keep them working for months at a time. With fixed knives, you don't select just a cutting height, you can select an operating DEPTH!
The forestry versions are the heaviest mulchers I've ever seen in use. Feed them 125hp and they'll grind 6"-8" stumps off at ground level. They can do more, but you either need a super-slow creeper or bog down your tractor over and over. Feed them 250hp and they'll grind whatever you want. Almost bought one but I'd have had to buy a 150+hp tractor w/creeper and spend a minimum of $40,000 (the really big heavy ones are $90,000) on a 225cm mulcher to do what I wanted to do.
If you're mowing lots of rocks with a mower whose cuttershaft rotates counter to the tractor's tires, make sure you've got a good screen protecting your rear window or head, especially if you're using hammers and the front lower shroud of the mower gets damaged or bent a bit. Those knives or hammers will scoop up rocks or shattered bits of rocks and fire 'em forward and, yes, upward. Don't ask...
