You have this backwards. The only disadvantages are extra space taken by the frame and extra weight.
They are not less stiff. If this were true then pickup trucks with higher towing and cargo capacity would be unibody.
This is incorrect. Race cars are optimized for low weight, but with extra rollover protection added. They cannot tow or haul much of anything. And are not street legal with crumple zones when built for track with tubing. Additionally unibody street cars/SUVs don't have a 3 dimensional steel tubing, cage style frame.
You are right about extra weight, but they provide a superior basis for stuff support. Ever look inside a unibody engine bay? Most with decent engine power, can't even keep the struts from flexing without a support beam going across the engine bay. Most are very susceptible to road hazard damage, and again, if this were stronger it would be used on pickup and other larger trucks.
Largely because newer isn't necessarily better if you care less about fuel economy than longevity, low repair costs, and lower purchase price.
I haven't even mentioned the problems that arise once they start rusting. Many unibody SUVs need special strut tower reinforcements added once they rust out. If there is not a kit then you are left DIY fabricating one. It seems unlikely that vehicles sold new today will have as long a viable lifespan as those sold 20 years ago.
There are advantages to more modern designs, but they are primarily greatly improved handling and weight:size ratio reduction.
I should have phrased it a little different and should have been more precise in my arguments.
If you look at the weight and the material that is needed to achieve the necessary or desired stiffness of a structure, then a simple frame with a body mounted onto it is actually a very poor design approach and leads to a very heavy structure.
But because trucks are built so heavy, their frames then do of course end up being stiffer in the end than a lightweight modern design often does.
But the structure of these fat frames inherently does not provide much torsional stiffness around the central axis which plays a major role when it comes to handing in turns or off road. And to compensate for this they need an awful lot of steel and end up being awfully heavy.
Modern designs achieve much better torsional strength with much less material!
Especially the example of modern race cars with their tube frames shows that. These steel cages of sports and race cars are actually not so much only designed with rollover protection in mind but much rather with enhancing the torsional stiffness to improve the handling in high speed turns. Twisting of the body directly and negatively affects the suspension and the positioning of the wheels and tires on the road.
For trucks of course that isn't quite as big a priority and issue as it is for modern sports cars, but still it does have an effect on off road performance and a trucks handling in turns. But with bigger engines and demand for better handling of modern trucks and especially SUV's this naturally does start playing an increasing role just like it did with sedans and sports cars many years ago.
For a modern SUV something like a "beefed up" heavier designed version of the Aluminum space frames could for example be FAR superior to the classic separate frame and body. But it would end up costing the car makers more and reduce their margins for profits unless they can raise the price.
Now for towing heavy trailers and maximum towing capacities of course a relatively heavy weight of the vehicle actually even provides a certain natural advantage. So that also plays into the trucks design of course. The higher the towing capacity needs to be the less the need for reducing the vehicles overall weight.
Old cars also tend to be built much heavier and sturdier than most new designs, but that is not due to them having a unibody. That is much rather due to modern cars being designed with quite different priorities, primarily meaning reduced weight, improved fuel consumption and much better safer crash behavior in case of a collision. And a modern unibody design is in many ways superior to the classic frame with a separate body mounted onto it.
On our 2nd gen. Explorers for example the whole body is practically not much else than basically just a classic passenger cabin and doesn't provide much if any additional stiffness to the frame and suspension mounts. On integrated unibody designs all that metal in the body is part of the entire structural design and even the windows are glued in and come under stress providing part of the overall stability and stiffness of the vehicle.
And I'm very confident that the new unibody design of the new Explorer for example provides a considerably stiffer and lighter platform than our old 2nd generation ones with the separate frame and body have. (Same for the BMW X5 from 99 and later, by the way, the first SUV with unibody design on the market.)
But yes I agree new cars and SUV's most certainly won't have as long a viable life span as our 2nd Gen. Explorers, but in many countries they also don't really "need" to. Many other countries other than the US, especially those all over Europe have strict regular technical inspections and emissions tests. They have very high taxes on owning and operating old and polluting vehicles, much higher prices for gasoline etc. So in many places driving an old vehicle is not as cheap as it is in the US.
And with time the US auto market will most certainly also move towards that direction as well.
And with the increasing number of hybrid or even all electric cars as well as the considerably high costs and short lifespans of their batteries, the aspect of a vehicles viable lifespan is most likely going to get dramatically worse.
