Vantaggi
Tuenti engineering culture is one of the best you can find in Spain. There isn't too many companies that use what I usually call "American Style" when running a Software company on Spain: software developers have a great impact on the decisions and the way the things are done, there is a clear career path as an engineer (no need to promote to non-coding positions if it is not what you want) and the quality of the people who work there is far above the average.
It had helped me as well to have a better CV and a great professional network (lot of the engineers that left end up in other interesting companies like Facebook, Google, Spotify, Atlassian...)
Svantaggi
IMO Tuenti is slowly going down as a company; their challenges are not that attractive any more. However, it is still one of the best companies to work in Spain as a software engineer, their processes, tools and knowledge still beat (by far) most of the companies on the country.
There is a lot of pressure and it is not expected to have a good your work/personal life balance. I'd recommend it if you are young and you want to learn and improve your CV. If you have family you will likely struggle a bit.