As always, that depends on your threat model. I'm definitely not an expert on the matter, but in the last couple of months there were a number of vulnerabilities in electron apps, including Slack, Discord, and Microsoft teams. The Signal desktop app is also electron based, so I wouldn't be surprised if some vulnerability was published in the next couple of months. Additionally, I have no idea how exactly the signal desktop app stores its data, and how easy it would be to extract it.

Personally, I'd stay away from the desktop app if I (or the source) were actively targeted by someone with enough resources to spend on either electron bugs, or other ways of accessing my computer. That includes spear phishing.