Lightweight aluminum frame transported via trunk rack
As far a aluminum goes, do this experiment. Get an empty soda can with no dents and stand on it. It will support your weight. Now put the tiniest dent in the side, notice how easily this happens. Stand on it and it collapses with a fraction of the weight it previously supported. 

How does this translate to bikes? A bike frame is built much much stronger than it needs to be to support the forces generated when riding. They are built to withstand the forces that will cause dents, the even a dent in aluminum can be catastrophic. 

When placing a bike on a rack such as yours, there is a chance it can be dented - probably not from straps being too tight, most likely from other bikes on the same rack banging into it. If you secure the bikes firmly, they are fine. You must use padding between them of some sort - "Dog Bones" are great (plastic spacers with a strap at each end), or foam.  If the bikes can swing around freely you damage them. 

I used this style rack for years with aluminum bikes, as do most of my mates. The main reason to discourage the use of such racks is every bike I carry has paint damage from rubbing against another bike where I was a little less careful than I could have been putting them on. I have seen derailleurs knocked off from them as well. Roof racks or rail based racks are much better at protecting the bike in transit and virtually ensure no forces are generated that will break a frame.