When I first started using GoodData several years ago, their features were inline with the best of the (very few) cloud analytics platforms. Their support team also used to be top-notch. These days, you can count on 3-4 exchanges, across 2-4 days just to 'clarify' the issue.
They have a proprietary query language that is difficult to use, learn, and get support on. That makes building even basic metrics a highly stressful, time-consuming process.
Their modeling tools are great for an extremely narrow, extraordinarily simple set of use cases. Many-to-many relationships are a complete disaster in GoodData. If there are legitimate many-to-many relationships in your model you should stop reading and run, as fast as you can, away from GoodData.
Their ETL tool is also locked down to their ecosystem, so finding help or support is extremely difficult.