One practical disadvantage compared to Linux is that it's not GPL, so if Google uses it to make a mainstream mobile phone operating system, other vendors won't be compelled to release their kernel sources like they are with Android today, which means a Fuchsia equivalent to LineageOS might not support as many devices.
Also, capability-based security is fine as long as there's some way for the owner of the device to acquire all capabilities. If there is not, it's fundamentally anti-user, since it's treating the user as an adversary. Since most Android devices require aftermarket modifications to get root access, I'm not too hopeful about Fuchsia. If these bullet points are anything to go by and DRM and verified boot are first-class features, it's likely that Fuchsia will have built-in mechanisms that prevent the user from copying protected video content, so it's unlikely that the average Fuchsia powered device would grant the user full privileges.