The downside of Signal is that it’s centralized, and thus at the whim of those who run it. Structurally, it’s not really different from Whatsapp or Telegram except for who owns it.

Mostly just that it’s still pretty new and thus hasn’t been as polished or scrutinized yet. Haven’t tried it myself. For the sake of the OP’s question, it may also be notable that it’s a UK company.

The two encrypted messaging platforms I currently suggest are XMPP or Matrix. Both are usually fine and are decentralized. The main thing with them is to either self-host or choose a server you trust to set up an account — which applies to the Fediverse in general.

The quote from the email isn’t her words. They were given to her (and all agency heads) to send out to their workforce to implement the EO. It should not be taken as “embracing the new regime”.