I agree with @Karolina and @Bashar that Quizlet might be best used as a pre- or post-lesson task. It is indeed a tool of polymorphic functions, which promotes students’ autonomy and independency in learning vocabulary. Comparatively, Socrative can be a fun tool for in class assessments, which can test not only vocabulary, but also can be useful for review lesson quizzes, lesson reflections or lesson warm-ups.
I prefer Socrative due to the fact that students get real-time feedback, performance results, but also the teacher can monitor progress during an in-class task.
I believe both tools can be used in an EAP context perhaps in different situations, so that there is variation and students have experience of both, which can make their learning more interesting.
As far as limitations of these tools, maybe students find it hard to connect the material they are already studying to the information they are presented with on Quizlet. Also, not all of the students will retain the information/words they have just learned – plus not all of the students are visual learners and retain information the same way. Although Socrative might be a bit more of a brain-teaser, if students are not prepared for an assessment, their performance will not be great and then perhaps the assessment will not have served its purpose.