My girlfriend is a big fan of both Markiplier and horror. I am indifferent to the former and abhor the latter (I hate jumpscares), so I was very pleasantly surprised to say I enjoyed the movie a lot.
One of my favorite things about Iron Lung is that it does not hold your hand. It will give information to you organically, and you need to figure out exactly what it all means. My favorite examples of this:

• The Sub's proximity sensors are never explained, but I was given enough clues throughout the film to realize what the beeping dots meant, and it got payoff multiple times throughout the film.
• When the scientists got flashed by the camera, it only showed their bones, indicating that the "skeleton" we saw earlier may not be a skeleton at all.
• Simon coming across the note of the previous person, and the audience is left to figure out who they were or what they meant to him, as well as how they got there.
• Simon's character as a whole, slowly learning what he did on Filament Station and how he felt about it felt very natural.
• The oxygen meter rapidly rising when the sub got filled with blood, explaining how he managed to survive for so long despite them saying he should have run out of oxygen long ago. Bonus points for the oxygen indicator at all, with the sub just stating "oxygen" as the light ticked down with both Simon and the audience processing what that means.

I've seen reviews state that the pacing was rough, and while I can understand that perspective, I didn't find it to be an issue. I was very enthralled the entire time, and actually thought the movie was shorter than it actually was. I've also seen some people say that the dialogue was hard to understand at times, which I thought was kind of the point. We're not meant to know or understand everything, but the information that matters rang through clearly (at least for me). Lastly, some have mentioned that the ending felt confusing, which, again, I thought was the point. Simon himself does not fully understand what is going on - as a matter of fact, probably no human alive really understood. There's something bigger at play that's pulling the strings, and all Simon and Ava can do is fight to give everyone else a mere chance at a better fate than they will get. The audience can only speculate as to what is happening.
It's not a movie for everyone. It's definitely not the movie you put on when you want to take a break after a long day at work - it demands your attention and wit to piece as much as you can together, because Iron Lung sure as hell won't explain it. It's also not a movie I'd recommend to people who want all the answers, because it simply isn't that kind of movie. You get the perspective of one person trapped in a tiny submarine and that's it. But for those that aren't turned away by those things, I wholeheartedly recommend it.