Here is what I think of this event.
First, those are not "Hidden" rooms, but randomly spawned areas, as you just have to hope there is a teleport beacon leading to one of them. You cannot miss it unless being blind or rushing a mission.
Second, it took too many hours to finish this event. Around 50h, including a whole Void Crusade where there was only one Hidden room (Emperor was maybe looking somewhere else).
Third, only the Favour were really valuable rewards. Everything else I got was either less interesting than my current inventory, or not corresponding to my playstyle. Here are the final rewards I got:
Some Psalm Codes... better than nothing even if I already have many in my inventory.
Some Shards... again better than nothing, and again I already have many in my inventory.
Then, some weapons I don't need or weaker that what I have in my inventory or stack. How lucky I am ... but I never use Bolt Pistols as my Sniper only uses Snipers and a Boltgun. The game should check what might interest you, to give you something valuable instead of really random stuff you never use at all.
By the end, the best rewards I got during this event, is to be able to buy the Honor Banners for my ship (previous event was for the Reliquary), and the Crimson Flames of Faith frame. Note that the Imperial Aquilas looks the same than the ones we can see in the maps during mission, and should be more unique for our ship.
Spending so many hours for such small rewards is unfair, especially when you see that playing with a another character in Story Mission means 0 Hidden Rooms (never got a single one after playing several Story Missions), which means you have to keep using your Main who finished everything (or ignore the Story... nope). That, plus the fact you must also ignore missions using open areas, as they contain no Hidden Rooms. Far too restrictive as said in other topics.
I prefer the Edict of Termination event, that was more interesting (special enemies VS "hidden" rooms) and faster to fulfill.