My second build ever (first one i made was a full tower ATX back in highschool like a decade ago) but this is my first Mini ITX Build and BOYYYYYYY did i have some issues.
I thought I did my research and was able to fit the H100i GTX cooler in my case, which I could, HOWEVER, with my current motherboard, the hose coming off the radiator prevented it from seating completed. This prevented me from fitting that side of the radiator into the case without breaking something off my motherboard, it kept me from mounting the lower brackets that held the fans in place, and it prevented the side panel from closing.
I was so frustrated....BUT I NEVER GAVE UP!!!! I remounted all of the fans, completely flipped the radiator and had the water lines exiting towards the front of the case (which is not the way you are supposed to install it). They said it couldn't be done and i would need to buy a new cooler, but with some shimmey shimmey here and there and some SERIOUS cable rerouting/management and LOTS AND LOTS of pressure, I was able to fit the radiator in the PC and place the optical drive bay correctly.
It came at a cost: 1) I couldn't mount the lower bracket for the radatior fans cuz the case wouldn't close so I had to just leave it off and secure the cooler with just the top mount, it worked fine and the case closes without bulges 2) The case is so tight and the radiator is so big and hoses are so thick that there was barely any room to remount the front chassi fan (140mm). In fact, there wasn't any room cuz the hose was blocking the corner of the fan, so I cancelled the 200mm fan i ordered to replace the 140mm, took out the 140mm and replaced it with a 120mm fan (sad face). EVEN with the 120mm I still couldn't mount it correctly using the screw holes cuz of the huge freakin cooling hose so I went the super ghetto route and mounted it via zip ties.
I know.... i know.... but hey, the thing runs great! So for all you H100i GTXers using this case out there. DOUBLE TRIPLE QUADROOOOOOBLE CHECK that your motherboard won't interefere with mounting. Or go straight up YOLO like me and do whatever it takes to get that sh*** in there! :)