The hate on ISS comes from a number of reports of hoops being pulled out of round by them. The design is inherently very clumsy - attaching a mount to two points on the tension rods can put extra tension on those rods and the head which other than pulling the hoop out of round, can affect the tuning. I would much rather have drums that are in tune than drums with suspension mounts.
The problem isn't so bad with thicker hoops. Die-casts would probably be fine with it but with the hoops that are usually used on mid-level kits like the Vision, then they are susceptible to being pulled. I think there are much better mounting solutions out there and I think the ISS is fundamentally flawed. Pearl really should change the system, they've been using it for years now and it was never a good idea in the first place. Apart from anything else, I think they're visually hideous.