The SSDs on the new iMacs absolutely have full S.M.A.R.T. support. Disregard what System Profiler may or may not say about it.
The problem is that most of the current S.M.A.R.T. utilities (and Smartmontool) apparently do not yet have support for NVME.
DriveDX has a beta with NVME support which I am currently using. If you own DriveDX you can write to their customer support about it and they'll send you a link.
OWC has a program in beta called Smart Alec that also supports NVME SSDs which you could sign up for but it isn't nearly as elegant as DriveDX.
So the conclusion is that in Sierra an NVME SSD in the iMac 2017 shows not supported and hopefully it will be solved in High Sierra and nothing is wrong with my computer =)