The most robust form of mudguard, even alu despite being very light, can crack if mounted incorrectly.
Plastic guard like SKS are more of a compromise, doesn't really reduced side spray, the stay mount are on the inside (water pour out on the stay), heavy stay to compensate for the flexy nature of the mudguard and not long enough in some cases (longboard have excellent coverage but a lots weaker near the flap because the stay doesn't extend further down).
I recently got a replacement longboard and the front guard's bottom stay had been moved lower than on the previous two I've had (and had broken). Can't tell if it's made it much stiffer, I took the original heavy flap off and fitted a pair of raw flaps. Hasn't snapped off yet though.
The most robust form of mudguard, even alu despite being very light, can crack if mounted incorrectly.
Plastic guard like SKS are more of a compromise, doesn't really reduced side spray, the stay mount are on the inside (water pour out on the stay), heavy stay to compensate for the flexy nature of the mudguard and not long enough in some cases (longboard have excellent coverage but a lots weaker near the flap because the stay doesn't extend further down).