No it isn't hard to ruin a disc, not atall. Yes some discs are stronger than others, NORMALLY the more material the stronger the disc. The disc protector IS good, i had one in my last few months of running a disc and it did help alot, however the actual protector does bend and is hard to bend back (it hurts if you bend it back with your fingers (thats what he said )) It does take a fair few hits before the protector needs bending back though. However i guess the amount of hits depends on bike weight, obstical(spelling??), your weight and how hard you hit it.
Disc brakes are a massive advantage though eg: for riding places your not ment to ride, riding late at night, riding during bad weather, not having to let down your tyre when you take your rear wheel out, theres probably a few more i haven't thought about.
Disc advantages, bend the disc (obv) they're easily contaminated, you cant bath bleed your brake, they CAN be harder to set up, you mainly just need the disc straight and even between the pads though.