[kragen]

x can't run untrusted applications; it trusts everything

any x application can spy on everything you're doing in other windows, send them messages, inject other windows into them, delete them, and post ui events to them. basically with very few exceptions any x application has total control over your account

xpra has other benefits (lower network bandwidth usage, being able to reconnect after network outages, being able to move an app from one display device to another) but that's not what the grandparent comment is talking about