Because it is too often applied ahead of the B-pillar, which is unsafe. Because it is often applied too dark, which is tacky. Because it is often low quality, and has bubbles or comes off.

You can avoid all of those issues, using a slight tint that replicates factory tint, only behind the B-pillars, from a good quality installer, and then I have no negative comments. Trouble is, not many do that. I am one of those guys that does a happy dance every time I see someone getting a ticket for too dark a tint, or obstruction of the view of the driver. Again, that is just me.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue