European FinTechs I've used (CurrencyFair, Revolut and WISE) all seem to have this in common with DeeMoney. At some point, they've run into problems with some of their international partners, and ultimately they have had to switch.
That's why I wouldn't dismiss DeeMoney's explanation out of hand. I've been using them for years, but only for transfers to Cambodia, where they have a strong relationship with their local partner. Those transfers always arrive on the same day without fail. I don't think they wouldn't like to provide the same quality of service for transfers to other destinations.