it depends ENTIRELY on the game, they can either be laughably easy, impossibly difficult or anything in between. personally I've only been able to get one in PS all Stars since almost every trophy in that game is incredibly simple. like 60% of them are just "go to this stage as this character and use a level three super attack" or "beat arcade mode as this character" etc., both of which can be done with almost no effort if you choose the easiest settings....
there where some other games I came pretty close to though; most of the time if I'm about to reach platinum I always run across one trophy that just ends up thwarting all my efforts from being plain damn impossible. Dead Island was a good example, I got like 70% of the trophies just from playing it so much, but I couldn't go platinum because of six or so ridiculous trophies, I remember one of them required you to get like ten one-hit kill headhsots IN A ROW or something like that. just getting ten headshots in a row is difficult enough, how the hell would you get 10 repeated instant kills if even common enemies take like 8 shotgun blasts to the face before they die? the only way you end up killing them in one hit/shot is if you get a lucky critical hit to the head with an OP weapon!
you either love it or hate it. personally I loved the massive open world fun you could have and really enjoyed finding dozens of different things to beat the shit out of zombies with, other people might hate the terrible story, repetitive gameplay, and awful AI, and it numerous glitches too much to enjoy it. If you've got lots of time to explore it and find every secret, and some friends to play it with, it's pretty good, but otherwise I can't recommend it too highly.
Stay the hell away from Dead Island Riptipe though, it added absolutely nothing new to the table, cranked all the numerous problems I had with the original up to 10, it's far shorter and has virtually none of the original's charm. not to mention it runs slower than a tortoise dragging a piano up a hill on minimum settings, despite the fact that my comp passes even its recommended specs with flying colors. its an absolute fucking joke; the fact that it came out a mere month or two before the Last of Us is absolutely mindboggling.....forget they're not even within the same PLANET of quality