First, the side charging handles the replace the GI are different than actual side charging uppers. Second, side charging uppers with a handle attached to the BCG are different than those which engage it only when charging. The first reciprocate, ie go back and forth with the bolt, the second don't,
When the bolt handle moves it can hit things you might be leaning up against, so they aren't as good taking up expedient field positions. Better left on square ranges, doesn't mean the M14 didn't have one. You have to accommodate handle that can jam the action is all.
On a range rifle a right side charger can be a benefit when strapped up in the prone to operate the action with the trigger hand. Upright in combat, it would interfere on that side and slow you down - the off hand charging would be preferred for more rapid getting it back into action.
The AR/M16 was designed with a bolt hold open, so theoretically, you only charge it once and could go thru enough mags to melt the barrel. When that has been filmed, tho, the action eventually jams and it takes cycling the bolt to charge it. Since that is ammo related, the 3Gunners are toying with enhanced charging handles or even side chargers because they are on the clock and seconds count.
For the average user it's an expensive addition with very little real return. A $225 side charger vs a stripped A3 for $39 is a steep entry fee. The comparison is a bit tilted, most side chargers drop the charging handle which is $8, the port cover, another $8, some hardware, etc. but it's still a $150 upgrade which isn't necessary to the function of the weapon, and that's been proven in combat long ago. Most of the new battle carbine designs use an ambidextrous or reversible charging handle, the point being they don't focus that much on it - because they all have last shot bolt hold opens, too.
Otherwise it's $150 extra for zero return on reliability or accuracy. That's still up to the shooter to care for and maintain the gun with good mags and decent ammo, all three of which constitute the top three reasons for stoppages, mags, ammo, shooter.
It's the age of boutique builds even tho the clichéd term popped up nearly ten year ago during the M4gery craze. Now with billet, CNC, and low cost production we see it even more but there is still almost no reason to spend the money on it unless you have a very specific application that restricts you using the standard GI handle. Doesn't mean you can't - It's America, not saying you shouldn't, just understand exactly why you choose to fork over the money. All too many play games and drink the marketing koolaid over options, it's important to remember they make things because they can sell them, not because you need them.
Which is exactly how Rolex stays in business. It's not about wearing a watch to get to work on time.