Vantaggi
If you are working at the WBG as a genuine independent consultant (i.e. you have other clients, you work autonomously, and you're working on specific deliverables) then it's great. Good pay, interesting work.
Svantaggi
The WBG is absolutely shameless about using STCs as a "cheap workforce" . . . hiring them for staff roles, integrating them into teams, having them do the same work as their full-time colleagues, but calling them "consultants" and not giving them any of the benefits or protections that come with being an actual employee. They have no qualms about using STC as "cheap, easy" substitutes for full-time staff, dangling promises of eventual full-time employment as a carrot to put up with STC misuse and mistreatment. FWIW, this is super illegal in most countries (google "employee misclassification") but due to their immunities they cannot be sued for labor law violations (and boy do they know it!) Beyond this, stories of STC abuse, including wage theft, harassment, and retaliation are rampant. Again, if you work as a true consultant, it's fine, but if you're working as a de facto staff member, make a plan to get out within a year or two.