-
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).
Why steel mudguards?
Whats wrong with plastic or even aluminium?