Thats a rough question.
Are they hard? Your question is way too general. Short answer: some no, and long answer yes. They are not difficult if you know what you are doing. Some can be real tough, some are easy. As a rule of thumb just chalk them up as more difficult and stay away from them untill you have a mature tank with good filtration (over skimming) and strong lighting (mainly halides). But like everything in this hobby there are exceptions. Yes I did have a great mixed reef tank wth birds nests, montipora, montipora digi, and some others. It was in a 1og skimmerless nano with 96w of power compact. I did do 50% water changes weekly, I dosed calc and alk every day. Basically I knew what i was doing so it worked. I put the time in for a great reward so for me it was worth it. But if you want to know specifically if they are for you, sounds like no for now. Do a little research, start to narrow down your q's, and in timeyoull be askin the right q's. Post some info on your tank specs, thatll help too.