At first let´s be clear: This mission is great! It´s well designed, atmospheric and I recommend playing it to every Thief fan.

Is it like Soulfourge? This comparison seems obvious, so is it a Soulforge 2.0? I would say yes and no.
While it can be seen as alternative dystopic ending of the main game and resembles Soulfourge in many aspects, it´s different. It´s more about a key hunt and several areas are not easy reachable due to the devastation. Yes, you again have to sabotage something, but in a different way.
Design-wise it has it´s unique style, a rather dark mechanist scenario. The makers really did put a lot of effort into it. Some rooms look really stunning. Sometimes you just want to pause and enjoy the sight(but be careful that this one special enemy doesn´t appear behind you ;-) ). They also made some new soundtracks based on the original ones. Great!

There are also new types of enemies. And they are not to be underestimated! Of course they are bots. New kinds of bots. Remember the briefing video of the Thief 2 demo? See for yourself.

There is also custom voice acting. While the victrola ones sound decent, the Karras voice is...odd. When I heard it first I thought: Really? It sounded kinda off, a bit like a parody. IMHO not that great.

What about the overall level design in regards of solving it? Well, it´s a mixed bag. While it´s overall good, there are some things I didn´t like that much, especially after progress stalled for a while. Hidden stuff is overall ok - there are some secret doors that might not be so obvious first, but I didn´t find hidden levers.
Some ways can be very long, and awkward, so a bit annoying over the time. Especially the key hunt was not my favourite part - while not too complicated, it was one of the aspects that made me enjoy the mission less.
Especially considering that it is possible to soft-lock the additional objective on Expert(with some bad luck maybe also one of the main objectives - fortunately I didn´t do that) under some circumstances. The reason was that I missed a certain key - and then some of the enemies can open the door for which is it needed, so it´s possible to sneak behind them and go through it. But then it might not be possible to come back if you deal with them as going the other way might be complicated. So be warned about that. While it can also be seen as me being careless even other difficult missions are forgiving in such regards(like "Into the Odd").
Like said above, overall I still view it as a great mission, in my opinion the 2nd best of the contest(but due to that issue I came to the decision to favour "Into the Odd" over it, no hard feelings).
Anyway, IMHO this issue shouldn´t be necessary - maybe future version might add a 2nd key somewhere, as I don´t see much reason to keep the certain sections locked and non-pickable.

What about enemies? There is no killig restriction(like in Soulforge), so you can even go all the way blasting(it´s not that there are many humans anyway - masked mechanists and this one...special guy). If you manage to pick up a secret weapon, it is however much easier.
Anyway, even with the standard stock and the right management it´s not that difficult to blast away all the enemies. Fire arrows, mines, and something that works like Holy Water are your means to fend them off.

For size and playtime, be aware that this mission is huge. At least 3 hours for the first playthrough might be needed.
Overall, despite the flaws it´s an awesome experience, and I would recommend it.