This was my experience 6 months ago (I ended up using multiple firefox profiles for months because of it).
Just tried it again this week, and I haven't run into the "derp derp derp one half of your accounts aren't allowed to see this"-type error yet.
Great, will give it another go, in that case. There's one little niggle still. Signing into a delegated Gmail account, there's no option for Labs. I want my signature to appear above the message I'm replying to, but this doesn't seem possible.