Not knowing the exact articles/info you are referring to, I can venture some guesses at best.

Flux... Well lets be honest. Most people can be lazy about things and not clean the residue well, simply do not know the best way to clean flux, or how to know if it is clean. Couple that with the fact that people can get over zealous and use too much, and you can have potential issues.

There is also the possibility that they were never taught to use flux, and/or never used it themselves. This can lead to the "I got along fine without it so far, why would I need it now?" thing. Which can be related to the amateur trap of "I am just as good as a trained pro".

Silver solder... Hype... pure and simple. They get caught up thinking that the superior electrical properties of silver solder vs lead (and other) solders actually means something to us in the RC world.

To put it bluntly, yes silver solder is better electrically but the difference is not enough to be any help to us. For a highly sensitive IC circuit yes, those IC chips in some precision equipment might actually benefit from silver solder if they are sensitive enough. For the uses that we in the RC world will use it for, it make no discernible difference, but people believe there is a difference and they then think they can tell a difference when using it... (self delusion, its common for situations like this, its similar to the placebo effect)

Here is the deal... The military aviation specs are very tough (well they was when I was in) They changed them just before I got out to make them more in line with the industry standards. This actually relaxed the standards. NASA standards for space flight are extremely tough. (the standards I was trained to are equivalent) They are tough because reliability is paramount on a space mission. Lives may be on the line, and even if they are not, it is practically impossible to send a repair crew to some space probe out orbiting Jupiter.

So if lead solder is good enough for them, then its definitively good enough for us. On top of that, it is many times easier to work with and get good results with.

If someone can provide hard data showing significant gains between lead solder and silver solder on the same system (not identical systems, as manufacturing variances between them can actually cause a difference measurable with equipment) then I will gladly provide instruction on its use. (I will need to brush up first, as it has been years since I used any)