Unless the mod has a "non-removable" battery - I wont charge them inside the device. I have before in the past. Basically while I was on the road. Do I recommend it? No - BUT - if that's all you can do...
As for the types of devices I charged on the road - NEVER my RX-200's. Why? Because the batteries DO NOT 'USUALLY' CHARGE EVENLY! Granted, you can tighten down a screw that will allow them to get within 1 / 100th (.01V - .02V) of a volt of each other BUT it is still NOT perfectly balanced. IF I go anywhere - I'd trust my Reuleaux DNA-200 a bit more to charge and go. Which is why I take my DNA-200 everywhere. It does a BIT better with charging things evenly. The onboard charging capabilities with the DNA-200 are pretty good. The mod DOES still get a bit warm - but nothing to bet overly concerned about.
The thing with charging via USB vs External Chargers is this: Batteries can and WILL fail if the conditions are right. If you ABUSE them (OVER Discharge / CHARGE), if they're DEFECTIVE (Broken SLEEVES / Insulators), or you're negligent with them (tossing them around next to coins, keys, etc.)
I have YET to have a battery fail on me. My 26650's have been overdischarged before - I tossed them and bought new ones. So... what method of charging my batteries do I PERSONALLY use? Well that depends. If I'm in one place - I pull the batteries out of the mod and use and external charger. Once they're done, I pull them off the charger and put them away to relax for a bit. Taking batteries off a charger and dropping them into a device is another big no-no. The batteries may be warm to the touch which means they may be 'stressed' from charging, they may be getting ready to fail, or they may just have come into contact with the sunlight or something else that made them warm.
After a few months and a few cycles of charge / discharge - I hook my fluke up to them, test the final voltage in / out. If the battery sag is too high (3.7 dropping to 3.0V or more on fire) I usually toss them and replace them. I do have an in-line meter as well, but I don't usually use that too often. If a battery explodes in my charger - I might be a bit ...... off - but it's better to have it explode in my charger than in my mods.
As far as the Charger(s) I use: Nitecore D2's and D4's. Why? I don't need the bluetooth capabilities on a charger to check them. If I put the battery in and get an ERR code (granted the battery is installed properly in the charger) - that's an indicator that the battery is DEAD and it's time to be replaced ASAP! I witnessed someone trying to charge a battery that gave an ERR code and shortly after they dropped it into their charger - it ruptured.