As previously mentioned most mower blades are made from medium carbon steel... 10B38 actually. It has a high boron content which allows easier cold or hot forming but can be through hardened to that magical HRc 40-45 tough range. Why don't the OEM's use something harder? They would except most equipment needs to pass the ASNI B71.1 stake test in North America and a similar CE test in the EU.
It is a devastatingly destructive test where a mower is placed in a lab with cardboard "target" paper surrounding the machine. The mower is run at WOT and a 1" steel bar is fired up from the floor remotely into the spinning mower blade. After the huge BANG... (engine is snuffed instantly) the target paper is examined for any marks that may have resulted from objects (blade parts) exiting the mower. Anything flying off the machine hitting the target area results in a failed test. Any parts falling off the mower onto the floor results in a failed test. Failures are a pretty big deal. The stake will have a large V cut into it and is often bent as much as 30 deg. The mower deck gets it worse. A mechanic usually has to pound the spindle pocket back into shape, install a new spindle/blade assy, and the test is repeated for the other spindles. It is a 'destructive' test as the deck is almost always scrapped afterwards.
Several things concern me about Meg-Mo blades if I was a pro cutter or put a mower out anywhere besides the "back 40". Harder blades (north of HRc 45) have been exhaustively tested by OEM's and are very difficult to get past the stake test. The swing blade design probably helps blade fracture during impact but I would still be mindful. Also, a very common issue when a mower blade is impacted is that the other blade(s) unscrew themselves and falloff due to centripetal force.... often mangling the mower deck as they fall. Given the high mass of the Meg-Mo system, blades unscrewing themselves would be a high probability. All concerning but the stopper for me: adding 4 new blade joints into each spindle. With many mowers exceeding 18000 fpm tip speed, the consequences of a blade fracture or joint failure is just too great to nearby people, property, or my machine. I'll keep sharpening and swapping blades as necessary thank you.