They do look good on the 71 thru 73 model year fastback...and they are very practical...helps to keep the sun from destroying the interior. Darn rear window on the fastback makes it more like a greenhouse!!
As far as the trunk lid hitting the louvers, it's been many years since I had a set, but I never encountered any interference when I did have some on my 72 fastback (sorry, I don't remember the manufacturer). Just make sure you don't get cheap when buying them (stay away from J.C. Whitney!!!), go for quality and ask at car shows about the manufacturer, or call one of the Mustang parts stores (Paddock, Year One, etc...) and find out who they would recommend.
I think if I did have to come up with some kind of "negative" for the slats...in the fall the leaves and pine needles love to get up underneath...so make sure you clean 'em good and clean 'em often!
And as far as hitting object when backing up with a fastback, I agree...that is just plain bad driving. I learned to drive with a 72 fastback, mirrors are small, huge blind spots, rear view mirror virtually useless....but I never, ever hit anything while backing up, and I drove that car for 5 years....great thing about the fastback, my friends always wanted to "borrow" it...until that sat in it! Couldn't see out the back, trouble seeing over the mile high dashboard, and the loooong hood...God I miss that car!! If all goes well, this is the year I start the build of my yard ornament...a 71 fastback!!