I finally finished the IWM Campaign (and got the related achievements), it took me a while to get to this cause I put off the challenge achievements but I did get to it in the end. Crazy, right?
Overall, the campaign is pretty well made with pretty balanced gameplay, no complaints there. I had a lot of fun, and the bosses were also pretty balanced and enjoyable.
In terms of gem placement, the only gems I didn't really like are the ones that are just hidden behind walls, with various levels of clarity when it comes to the cues on how you find them. They were reasonable to find, but I feel like they are the lowest effort type of gem you can design, so I'm biased against them.
P.S. Also has Williams bonus content!
Credit for the thumbnail Easter egg you may or may not have seen: https://www.deviantart.com/thomasands...