I don't find Cyrodiil boring, just frustrating at times from the technical issues it often has. It's the only place in ESO that truly feels open world/sandbox like to me.
The PvP aspects of it tend to offer more dynamic and ever changing gameplay. I like to mix it up ever time I go in there but I could see where if you did the same thing every time you played (zerg'n) it would get boring.