Signal is not great on multiple devices (it works, but not always seamlessly), but there's a reason for that: Signal is also generating virtually no serverside metadata. There's a real, practical security win for the inconvenience it's generating for people who want multi-device messaging.

Compare that to Wire, which has much more convenient multi-device support, but accomplishes that at the expense of keeping the entire active graph of communicating Wire users in plaintext in their servers on AWS.

Both Wire and WhatsApp (which have comparable security models with some marginal tradeoffs --- WhatsApp metadata is in Facebook's custody, which is not great, but Wire doesn't have Facebook's extremely competent and well-resourced security team) are reasonable choices for secure messengers.

Telegram is not.

Yes. I continue conversations from my phone -> computer (and vice versa) every day.