Are Wi-Fi 'transparent' buildings superior to antennas mounted on the exterior?
"Wi-Fi" friendly?

As with any engineering task, you must first precisely define your objectives. 

In your case, to be "Wi-Fi" friendly probably means that the users of your space get a consistent and "fast" connection to the Internet (or other network resource). Bandwidth (communication capacity) is a shared resource. All users of your space compete for it. 

In a high density environment, like a hotel or office, it may actually be advantageous to use Wi-Fi absorbing materials. This allows you to reuse the same communication bandwidth in different physical locations.

This is how the cell-phone network works.



Basic concepts

There are really three things a material can do to a Wi-Fi signal:


Nothing -- Signal transits without alteration
Attenuate -- Signal is absorbed by the material. This prevents (or reduces) communication on the other side of the material.
Reflect -- Signal is reflected from the material surface. This prevents communication on the other side.


Real materials do all three in differing combinations. 

Application

"Wi-Fi," in common usage, refers to the 2.4GHz ISM band of frequencies (there are other Wi-Fi bands, notably near 5GHz). That frequency band is near the natural peak absorption of water. 

Accordingly, Wi-Fi will not perform well in moist environments (indoor pool decks, spas, near water pipes, etc). However, absorption is typically fine. 

Reflection is the bigger hazard in infrastructure planning because it results in interference. It's like trying to have a conversation in an echo-producing cave. The more you talk (and the more people talking) the harder it is for anyone to be understood.

Large flat grounded conductive surfaces (metal, most oil-infused wood, metalized plastics, foils, etc) are the worst offenders in buildings.