BMW Group has not advised anyone to tint - or not - their sunroof, or windshield or side windows, on the iX or any other models they manufacture, in any written or verbal communication as far as I'm aware. Ever. A dealer - or a detailer - may have advised someone not to tint their windows (or sunroof), mainly because of their lack of knowledge about their process or their product, or because they may have damaged someone's window in the process of applying same. Tinting - or not - is a personal decision based on 1) esthetics, 2) legal restrictions or 3) need. There's really no need to tint your windshield, for example, because it's safety glass (with a sandwiched layer of polyvinyl butyral between layers), and it already has a very effective IR coating, so it rejects almost 100% of incoming infrared and UVA rays. For esthetics, perhaps, if you want a darker windshield, but in the US tinting your windshield below a certain area (generally about 4" below the top) is illegal. Tinting your sunroof with a suitable tint will reduce visible light transmission and some heat (the layered electrochromic glass already rejects most UVA), and your tempered glass side windows - even the factory-tinted rear passenger windows - have almost no UVA rejection so tinting these will substantially reduce transmitted energy.
Any window glass can be damaged by the application of tint by a sloppy detailer, and a small chip or crack can propagate in tempered glass (all of your windows except your windshield - the sunroof is layered tempered glass). Window glass can have manufacturing defects that only appear some time after installation, and cause an ultimate failure. But none of these are related to tinting, and thousands of cars, including the iX, have aftermarket tint applied pretty much everywhere, with no verified reports of failure related exclusively to tinting, after several decades on the roads collectively.