I once had to go EC261 on United as they stranded me in London.
It literally took 18 months and they denied everything and said their cancellation was "force majeur " and I was entitled to nothing. Until the courts ruled and it was time to pay up, then they tried to bribe me to drop the case. I told them they were in violation of law for contacting me directly instead of my attny and hung up.
There are actually firms in EU that do nothing but prosecute EC261. I think they took about 15% and I had to do nothing except email them my boarding pass and other info. Not bad, but United was just appalling about it.