The ability to track candidates, manage resumes/CVs at one place - it makes life easier! I would say that there are some minor issues like having an app only option instead of web interface when you need more control over your data (e-mail alerts etc.).
Some time ago they moved away form cloud services so now everything needs be done via API's but this could also make sense as well since not everyone has access or knowledge about coding languages needed while connecting our system through APIs anyway :) We've been using them during last four years without any major problems apart few bugs here &there every year fixed quickly once reported properly within 24 hours response times were always good tooo!!