I would like to know expert opinions about if is possible to use solely ferrography and viscosity as the only tests in a used oil analysis program? What equipments and industry could be covered with only these two tests?

I would not rely on an analysis report not showing AES wear metals. This is particularly true for monitoring engines, gearboxes, hydraulics. Also, giving an accurate diagnosis without an idea of water contamination (by Karl Fischer or FTIR) on any kind of machinery is very difficult. When monitoring turbines and hydraulics, TAN analysis is essential to give an estimation of lube remaining useful life.
I consider ferrography the most powerful tool in lube oil diagnostics, but I can hardly think at it as a routine test. There are many other tests (AES, particle count, RFS, Lasernet Fines), which can tell you when it's worth performing a ferrography.
IMO, viscosity measurement is an essential diagnostic test only in the case of engines or other machines that may suffer from volatiles or heat contamination. In many systems (e.g. turbines) viscosity change is a tardive signal of oil degradation. When viscosity changes, often many other parameters (TAN, IR oxi) have already passed critical limits.
Good luck

ICP, FTIR, etc is an ideal tool for trending results and ideally should be used on a regular basis. The limitation is its inability to see above 5-8 micron. Here is where ferrography comes into a class of its own. Ferrography is also ideal for one off tests to determine the wear mechanism and severity that may be occuring.

Duarte,
I can hardly think of a single oil today that does not have a load of additives added. Because of their importance, their presence need to be tracked. Neither measuring viscosity nor using ferrography would give you any such info. Yes, viscosity is one of the most important characteristics of any oil, but sometimes you can have perfect viscosity, yet the oil is junk (no additives left). Ferrography is handy to separate other contaminants from wear particle, and help you identify those wear particles, but will not tell you much about AN, BN, RBOT, EP ability, etc.