They actually lack side impact protection around the legs and hips. I have a metal frame in my Graco car seat. The only benefit of having a metal frame per the experts, is that it reduces the flexibility of the car seat whipping forward. The thesis is that, when tethered, the car seat's frame has the same amount of restraint if it was made of plastic or metal.We love the Diono's. The Rainer is a full metal frame and they are slim. The seat can be folded up for storage/travel and you could fit 3 across in even a Jetta or smaller car if you needed to.
They are a little pricey but in my opinion totally worth it for their crash/safety ratings.
I forget which group, but someone started side impact crash testing car seats and some failed miserably.