There is no single central #Signal server. Signal uses #AWS cloud, plus Google Cloud, Microsoft Cloud and Cloudflare, all under US legislation. If any of these clouds goes down or becomes otherwise problematic, chatting degrades or fails.
With #deltachat you can choose #chatmail relays ( https://chatmail.at/relays ) which distributes the risk across a wider network. We are currently working on the multi-transport feature to finally remove any central point of chat failure https://chaos.social/@delta/115362144863345662