ok both mods are designed to be safe using the usb to charge
heres the skinny of it all
way back...most of these charging boards in a mod sucked. nowadays they are much better
but heres the things you need to pay attention to
you need to make sure teh cable is good
and that the wall wart(plug) matches your device. look at the manual and find the charging volts. your wall wart should match that and never exceed it. the amps or milliamps should be at least the same but the wall wart can be higher. so in the case of the charon its 5v/1a so the wall wart must provide 5vs not 9 and at least 1000mah(1A) it can provide 2000mah or whatever but has to be at least 1A (1000mah)
now the next thing is never leave a battery charging unattended regardless of using a mod or an external, shit happens. my external charger melted the surge protector one day, because I put a battery in backwards and its reverse polarity protection failed (thanks nitecore)
next. and here is why i recommend external charging regularly and to use usb as needed like when you dont have a spare set of batteries,
what if something goes wrong. i just gave an example of something that no one else I have told the story to has heard of happening, but it happened, and it should not have happened PERIOD. so when charging batteries shit can go wrong even after it not going wrong 4000 times before.
so if shit happens while charging the batteries in the mod, you got a higher chance that you damage the mod as well as the batteries. so now you have to replace the batteries and the mod. in my case, i just needed a new charger, and I could use usb charging to get me through till it arrived
next issue with usb charging in the mod, those usb ports are finiky at times, they are often prone to breaking easily. so if you dont have an external charger and you break your usb, well your fucked.
now im not saying any of this bad stuff will happen to you, but it happens, and it happens at the worse times.
so because of all this i recommend using an external charger, have 2 sets of batteries for each mod
get good batteries
sony,samsung,lg
and a good charger
xtar, nitecore, efest, hohmtech
get them from a reliable source, illum.com liiowholesale,com(there are others, but that's off the top of my head).because there are fakes out there
but your mod should not just blow up because you are charging it with the usb, BUT if something goes wrong, you use the wrong wall wart, you can cause damage to the batteries and/or mod and worse case scenario, yes it could go boom