I think both Session and Element can be used on multiple devices across platforms and are included as “additional options” to the main (more secure) recommended messengers. Security concerns have been raised with both Matrix and Session and it’s unclear which would be preferable.
@Niek-de-Wilde said in January that Privacy Guides will most likely be delisting Session, whereas Element is being kept on since it apparently isn’t affected by the same security issues as other Matrix clients. I’d be curious if @soatok had any comment on Element vs Session. All other recommended platforms have limitations on which/how many devices they can be used on, including SimpleX Chat.
FWIW Signal should work for most people: You should be able to add up to 5 desktops and iPads as linked devices and Molly allows you to use Androids as linked devices. If you have an iPhone, just be sure to use that as your primary device by registering your Signal account on it. The only time you should run into an issue is if you need to use more than one iPhone or if you use more than 5-6 devices. This setup might increase risk by adding additional parties to trust and potentially lagging a bit behind in updates.
I’d avoid Wire and XMPP if possible, they aren’t recommended due to security issues. If you’re going to use XMPP, be sure to use it in combination with OMEMO and understand its limitations on what it does and doesn’t protect.