Hey! Unfortunately my answer arrives late for you, but I hope it can be helpful for others who are also considering UC3M's Master in Economic Analysis.
I am a student in this program and I couldn't be happier with my decision of coming here. I have first hand knowledge of other similar top European graduate programs and without any doubt this is among the best. It is a very rigorous program with high quality courses. This is true especially in Econometrics, which is its main strength. I have personally compared the material covered in my Econometrics courses with friends from other similar programs and theirs is just a small part of mine. For example most programs do not get very deep in Measure Theory, there are some which do not even mention it, however we covered it in depth from the first days. The pace of classes is very intense and there are assignments every week in all the courses, so the workload is considerable. However, this forces you to give your best from the beginning.
The class size is small, between 15 and 20 people in the first year, therefore professors know you personally and it is very easy contacting them. All come from top level PhD programs and they have always been happy to help us. The same is true for the TAs, who are mates from the fourth and the fifth years. The general mood is great and you feel integrated in the department from the first moment.
It is true that the attrition level is high. The graduate program is five years long, divided in two of Master and three of PhD. To progress to the PhD you need to pass three qualifying exams at the end of the first year. There are two attempts to pass them and only few people, around three, are able to pass them in the first attempt. After the second attempt about 8 people out of 20 are admitted to the PhD. However, most of the people who do not progress to the PhD apply to other top schools and are generally admitted without further trouble. This shows how well regarded the Master program is. As you can see it is very demanding but it certainly pays off.
Regarding funding, it is also very generous. Nearly all of the students receive funding, covering the tuition fees and about 950€/month. This amount is more than enough to live since the UC3M is located in the suburbs of Madrid, in Getafe, and the housing prices are not very high. A room in a shared flat here is just about 250€/month. It is also possible to live in the center of Madrid since train commuting from Atocha station is just 20 minutes. Also Madrid is a great city to live, even though you will not have too much free time.
This is a broad overview, if you need any further clarification about any other aspect I will be coming here from time to time so feel free to ask. My bottom line is that this is a great program, very demanding, but if your background in statistics and mathematics is strong and you are ready to work it should be your choice.