The main advantage of WordPress is the huge community. Created by an incredible number of different pre-made themes (both paid and free) written plug-ins for almost any occasion. On WordPress you can create almost any website (even you can do SPA, but the appropriateness of such decisions is questionable). You can Google the solution to 99.9% of the problems that you have when using this system. System almost survived many other CMS-Ki, and now, when the trend in the creation of every "gopher" to your CMS-Ki passed, it still strengthened its position (IMHO).
The world is changing, and the classical sites, the demand for them has fallen, as the cost of development (including through WordPress). And WordPress has become a classic (and almost the only) solution for creating these sites.
I think it is confusing and unclear,although I heard that he is a lightweight tool
Like any other CMS, WordPress has its own architecture, a template system, the admin has its own logic and interface circuit. It's all a matter of habit, and if you are comfortable in it, it will not be easy even to get into the source code of the theme and to perform a quick, deep customization to suit your needs.
The main disadvantage - all the same popular. For example, from time to time attackers find a vulnerability in some very popular plugin, and a massive "drop" sites where this plugin is installed (especially if you do not observe elementary safety rules, all the key points of the entrance you have a standard, plug-ins have not updated). Even if you have backups it may significantly poison the life if you churned out these sites "like hot cakes" in large numbers.