I have been using it for quite some time now without any issue at all, from its ease of use to security features! It has everything you would require in an excellent database system that can support multiple workloads seamlessly.
This product is just awesome with almost no downsides as well!! If there are then they must be very small which should hardly affect your daily workflow or performance issues faced by other products like MySQL etc., The best thing about this software compared ot others i've seen so far (like Teradata) is how good their documentation/tutorial section was when first launched - something not many DB systems offer these days. From my experience till date nothing bad came up regarding monetdb but if anything did come out then it's pretty easy fixable too unlike Oracle where fixing requires extensive knowledge base stuff. Data extraction through API calls & data analytics capabilities makes things easier than ever before since we had.