A Legacy style TTRPG is a fun concept - I know, I've got piles of printer compatible sticker sheets and lotto ticket style scratch offs waiting for my own project along those lines!
I enjoyed reading over this. A lot of the "Legacy" schtick is about intentionally placed voids in the campaign setting and mechanics, which has the fascinating twist of formalizing who gets to settle those bits of undeclared worldbuilding. It also lends to a paced escalation, not only of player/character knowledge but of the threat level of the world. Interesting stuff!
Having not played it yet (I'm a year into running a Dracula Dossier campaign that's showing no signs of slowing down) it seems like this would be well suited for a group with a GM who likes to doodle and think about factions and game politics without an excess of specific scenario planning, and Players who don't mind losing characters and making up world changing factoids on the fly. It seems like it would really reward repeated play, something in the 12 to 18 session range probably, because of how it's structured to enable callbacks that Players have buy in with.
The mechanics seem quick to pick up, resting somewhere between Black Hack and Apocalypse World. The introductory scenario is almost ready for immediate use, it probably wouldn't take more than twenty minutes to think up enough details and derails to make up one or more sessions of play. The biggest impediment to play is the necessity of printing out (and binding) (and bookmarking) the 100+ pages of the manual, which (because of the Legacy elements) can't be used digitally or streamlined. [Edit: But note the reply below]
I'd play it if offered a seat at the table, and I'd run it if a player asked.
(Mild Spoiler : I enjoy the meta-trolling on page 55 re the number of Unearthed Secrets check boxes)