Honestly, the only issue I have with tanks in general, is how aboslutely ineffective magic damage can become against them, since mages don't really have anything to fall back on (yeah, I main mages; so if my bias shows I apologize) like the 100% AD scaling autos of AD reliant champs.
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2310176716/209857063
Had this game a day ago on Xerath, even with 3 penetration items (Abyssal, Sorc Shoes, and Void Staff) I couldn't even SCRATCH Yorick even dumping 3 rotations into him barely had any effect. Surviving a multiple rotations is fine, but for them to barely even dent him seems absurd to me.
Am I just doing something wrong here?
Edit: Worth noting that I understand its the combination of health and MR making him tanky,not his MR alone.