Proton has the disadvantage of having to work with other email services as well, so there's protocol limitations. When mailing from one Proton mailbox to another, they do intentionally avoid SMTP for this reason, but Signal has the advantage of "owning" the whole protocol, too.

I imagine if you donate with a CC to Signal, they might also be forced to turn that over. The weakness is not in Signal or Proton, but in the Visa/Mastercard duopoly and CC processing in general. Cryptocurrency has some advantages here, but they are outweighed by the abuse, fraud, speculation, and general dishonestly (and just general failure to be good currencies for "normal" purchases.)