You're right that "two-handed," per se, isn't an advantage, but the problem is that it is a big-damage Dex weapon, which is highly desirable, since Dex is already overpowered compared to Str. Also the "1d8 is better than 2d4" situations you discuss are fairly edge cases, 2d4 is still generally better.
Also, in comparing the Double Scimitar to PAM weapons, you're right in that it doesn't get everything that PAM does but PAM is a full feat whereas Revenant Blade is a half feat, and it gives you additional AC unlike PAM. It is a strict upgrade to TWF, while also stepping on the toes of Str weapons in general.