Zbrush can be simple, yet complexed, depends how deep you go. The advantages with Zbrush is its a artest tool box, its fun, and there are tons of free training and tutorials avalible for it.
Pixologic have spent alot of effort with adding plugins to extend its fuctionality, look at the recent plugings that have been brought out, outstanding they are too.
Zbrush is not what I would call a 3d modeling program, not in the traditional sence. There is no camera, the object is what rotates in the scene. There is no quad views, so you cant see more that one angle while working on your model. The move tool is more like a tweak tool, while you dont manualy sellect Faces, edges or points like more traditional modeling programs. Zbrush has its own way of doing things, which is not a bad thing, but can be harder to learn if you have come from another 3D application. Zbrush was my first ever 3D program. I followed the manual, done the tutorials, and within no time was feeling comftable with it.
Reason why some use other applications, is that alot of people in the CG industry and hobbiest alike, have learnt 3D modeling in mainstreem applications that deal with a more techinical method of modeling. They are use to building the basis of there models in this way, to then bring them into Zbrush to add further details, then produce texture and other more advanced maps which get loaded into their application of choice for the finnal render.
Zbrush render engine again has it own methods, its not as easy to place lights as other 3D rending applciations, there are few rendering settings compared to other applciations, but can produce some great renders still, just not upto the standards of some other renders out there, its the artest that realy brings to life their work in either case.
Zbrush it realy geared at Organic modeling, sculpting, 2.5d painting, so this is not the application to make cars, bikes, athough some hard surface modeling can be done.
I wouldnt say Zbrush has any real dissavantages as its never tryed to do it all, or profess to do it all. For some its a pipline addition, while for others its a complete soloution for their art and charaters.
I would recomend you learn Zbrush while getting to grips with the basics of 3D modeling. I totaly recomend Zbrush, and if you wish to make the best of it, learn another simple more traditional 3D modeling program like Hexagon, I can help you further if needed.