I have heard "yes" and "no" but I don't like to vape personally via passthrough, even if a manufacturer states that you can. Like others, I agree it will likely put stress on the battery over time, if done consistently. I'd say you could probably do it in a pinch without any kind of explosion, but it's a habit I don't need, since I have some mod backups. I have a few "USB" charge batteries, a couple I really like, and I do find them useful on occasion. I also will only use a wall wart for charging as if something goes wrong, I don't want it affecting my computer. I also ordered a magnetic charger to cut down on wear and tear on my USB mods (and my phone, which is elderly, and starting to show wear and tear on the USB port.
So, it's not exactly "unsafe" it's just best practice. If you really like your smok stick there is little reason not to get a backup and swap them out, and it is probably good for your battery to "rest" some between charges.
If you want to go external battery especially if you like a higher wattage vape, looking at external battery mods is certainly not a bad idea either.
Anna