I have a 2001 ML430. Not sure when they did the major changeover to a unibody.
The 1998 to 20XX ML's were built on a very stout boxed ladder frame. In fact, an 01 ML430 just won the Paris/Dakkar race.
My ML never left me. It weighs 2.5 tons and has 8 air bags. My 16 y.o. daughter drives herself and her sister 16 miles away to their high school. I feel blessed that they have a very safe vehicle to drive to school on our chaotic So Cal freeways.
As far as reliability, I'd say it's only average. It's a V-8, so mileage is only around 15. But I cannot emphasize enough that it is a very solid, safe vehicle.
I don't know that much about the unibody ML's.
AFAIK, they're just as safe - but you might want to chech out benzworld.com!