Transformers are actually very efficient, IRO 95-98% as opposed to a SMPS which is 85-90%
Switch Mode power supplies can get very warm in use, but as long as you can keep your hand on the casing it's most likely okay
With any power supply, the best thing is to derate it, for instance if you need to draw 2A, use a 3A power supply.
Easier said than done though.
Linear have heavy transformers, are not as efficient as you appear to believe, so I have added a web site with the relevant data and comparison for you.
A properly design SMPS charger or power supply, will run at a very high efficency, wasting very little power.
Wasted power in a poorly designed charger, will result in it running warm, wasted energy.
Running warm/hot will result in a shortened lifetime for important components.
Modern SMPS units, will usually be designed around an industry wide chipset, which are usually cheaper, and easier to get the important parameters working well and are most efficient, and will therefore run cool as well.
An analog/Linear charger, is the complete opposite, running generally hot, and in general will have a shorter life.
Not recommended in this day and age.
Efficiency for SMPS is usually above 60% nowadays, whereas the Analog/Linear will struggle to achieve better than 30%
See here:-
The main difference between the linear power supply and SMPS is that linear power supply converts the high voltage of AC into low voltage AC then the rectification procedure takes place while the SMPS converts the AC signal into DC signal first then the stepping down of voltage signal takes place.
electronicscoach.com
regards
Andy