Well, as someone who has developed a shoulder/neck issue (also why I can no longer paddle a 5’6 for long periods anymore), it makes a significant difference. I wore a chest zip 2mil (otherwise same Isurus suit) last month and my shoulder was sore for a day after an hour session. It’s also the easiest suit to get in/out I’ve had. Third year for my 3mil and impressed by the durability. Feels like you're just wearing a rashguard. For me, it’s 100% worth whatever minor tradeoff for the increased mobility while paddling. But everyone’s needs and priorities are different.
This Isurus is night and day better than the only previous zipfree suit I had, RC Flashbomb+, about 8 years ago. That was great for 1.5 years but as others said, stretched out. As someone mentioned, I think the quality of the rubber makes the difference.