TL;DR; This mod belongs in the list of mods you absolutely have to play.
I am one of those people who like exploration and certainly gain motivation from a background story. My dream map would be something like Nauvis Post Collapse, but with a lot of existing factories spread out over the map so you can choose to either research your stuff, or try to use (and defend) existing factories (Nauvis Post Collapse has some but they are mostly incomplete, so completely useless until you research the things youirself, at which they become even more useless since you can build them yourself). This mod, despite not explicitly being designed for this, comes very close: almost all the missions have existing goodies for you to find or use, most of which are optional.
Having played this mod a while ago when it only had 6 missions or so, I decided to play it once more once I saw more missions are available.
I have developed tutorial maps for other games in the past, and making foolproof (or even "working in most conditions") maps is close to impossible, so I can feel the absolutely enourmous amount of work that has gone into these missions: making things "just work" in a complex game such as factorio is extremely difficult, and this mod pulls it off for the most part.
The individual missions are very varied, featuring standard base building (under various conditions, such as limited resources, limited technologies, limited space and so on), RTS-style building (where you have to build using robots), pure fighting with a few puzzles and more. Personally, I most liked the maps which featured building under constraints, and least liked the pure fighting missions, but I enjoyed all of them, and they are different enough to your normal factorio mod to all be interesting.
Story-wise, each mission has functioning goals that make sense in context and kept me going - they work pretty well standalone. The overall story arc is there, but... I couldn't make much sense of it, with missions not really fitting together and feeling more like individual vignettes in ther life of the engineer (which clearly they aren't). Despite this, if you enjoy factorio (which has almost no story or goals) then this mod has no trouble fueling your motivation with a (comparatively) rich background- and in-mission story line.
Difficulty-wise... I found it ramping up somewhat evenly, but brutally: If it says hard, it means hard. I am not a good fighter, and especially the higher missions can be brutal. Sometime s there is a way to simplify things (for example, by using efficiency modules to reduce pollution and therefore attack frequency), or maybe sometimes I found the right approach, and sometimes not, causing unnecessary difficulty, but the difficulty level mentioned certainly did apply, no small feat. This also means there is great replayability here, too, because you can successfully try out different overall approaches - something I wouldn't attest most other factorio mods, which, once you have a set of blueprints, are mostly "solved".
I also enjoyed the "even if its unfinished, we give you some end goal" attitude very fulfilling - you are not left hanging at the (temporary) end of the story, but you get a kind of bonus goal: In my earlier playthrough, this was eliminating the "real" enemy, far away on a map full with biters and no artillely, and in this version, it's a "build a rocket silo on a 128x128 map". Both of which are, by then, refreshingly different again.
In fact, this mod made me do what normally would be unthinkable for me - play without cheating in bots (because the beginning is boring) and even challenge myself by completing the 128x128 rocket silo challenge (feeding coal into my furnaces manually till the very end, but I did manage).
As a minor nitpick, it would be nice if, after fulfilling the objectives, you could continue playing - what happens now is that you can continue with the next mission or simply exit, in both cases losing your game and having to go back to an autosave.
Overall, this mod belongs into the list of great have-to-play-at-least-once mods such as bob's and space exploration, and fits into a very undersupported niche, namely story-based missions, which are practically absent from the mod portal.
In one word: Thanks!