Update :
this now seems to go better :
- on Monday 1st, I initiated a standard SEPA transfer
- on Wednesday 10th morning, I received an email from BTC-24 saying that the transfer was processed
- on Thurday 11th (today) afternoon, the money was on my bank account 
So the bottleneck was on the processing side of BTC-24.
I think I will continue to trust them at least for a while.