I don't understand at all why is this FM has so high rating, at the begging we start our jorney in the toilet, thats says a lot about our future gameplay.
First hours is completly chaotic and you can't figure out what's going on. I thought maybe I missed something , but everything that happend made no sense at all. In the middle of the mission I thought that Garret was sleeping and would wake up at the end of the mission but nothing happend, and it will make sense at this point that in the basement of the mansion is haunted cathedral with a black ghost with multiple eyes, a servant in the jar, a ghost.
Atmosphere in the underground is really good and it's very uncomfrtable to explore it for the long period of time, but again, whats the point of put player here and after that he again in the mansion. Maybe I didn’t notice where it would say that it was a dream or it was something different from a dream.
There is nothing extraordinary in the gameplay, same scheme : you have an objective=>you need to find a hint how to complete it => you find it and search for the object in the certain area.

I think the beginning of the mission is incredibly confusing and frustrating. Especially without your tools. The mission really opened up for me after I noticed the level was CHANGING as I played. Like the city was alive. I've never seen this in a mission before.

I quit and came back to this FM like 3 times before i finally save scummed my way through the beginning and got my tools back.

The very beginning of the mission is eyebrow-raisingly mean indeed (so much so I can 100% understand if anyone just decides to shut it down and give a 1/10 rating to the mission), but what comes after that has a special magic. I've got no clue what's happening either, the whole place is its own strange realm - and I genuinely think that is what fascinates lots of players. The (almost) dreamlike quality of the many places you can visit throughout the mission. Though if this doesn't resonate with you, I can see why the mission doesn't seem special to you.

Starting literally in a s*thole was a major initial psychological obstacle, as well as the lack of anything to ...defend yourself with. Things only got better (and better) from then.

If I remember this is one of Garrett's best close brushes with a powerful evil mage of any missions ever, packaged visually (and in mission layout, progression, IQ, all the small bits and uptouches, etc...) in a work of art.

Also on a wider general note, there should be an IRON RULE and pounding otherwise for anyone to only rate a mission once you have fully played and finished it at least once (if you have difficulty with the logical concept of a review/rating then at least out of politeness to mission authors).

"Is this FM is too overrated?" No.

It is where it is because it's technically great in every way, but also excels at giving the familiar along with the strange. After a million (greatly built) city missions that totally follow Thief's original aesthetic, something like this one is greatly appreciated by the community, IMO. In other words, it's originality in it's navigation/architecture and it's aesthetic.