The EUIPO is an administrative body, not a court — but its decisions have considerable legal effects, and they are adversarial: each party produces its documents, and the office decides.
The most striking feature of its regime is how open the revocation action is: anyone can apply for a registered trade mark to be revoked for non-use. There is no need to be a competitor, nor to demonstrate any harm. This is what allowed an Irish chain of around a hundred restaurants to mount a frontal challenge to the European registration of the best-known trade mark in the world — and to bring part of it down.