Dosent matter if it's ugl or pharmacy, it's a terrible idea. Yes some people backload syringes but I'm gonna list the 2 main reasons I feel it's a bad idea.
Carrier oils, many carrier oils can destroy the little rubber piece In the syringe. Mostly synthetic carriers but mct oil can also do this and most gear these days is made in MCT oil. Also if there are a bunch of solvents this can also happen. If that rubber starts to dissolve you're going to be injecting that dissolved rubber into yourself.
Contamination, once it's in the syringe it's not 100% air tight, once you break the rubber stopper a tiny bit of air gets in, especially if you use extra air to pressurize the vial. Then in the syringe a tiny bit of air can still get in through the needle. This all causes potential for contaminate.
With all that said, I'm sure some ppl still do it. But that don't make it a good idea.
If you decide to do it anyway then make sure your oil is something non corrosive to the rubber and after you've loaded everything I would individually put them in Ziploc bags or something to further ensure less air getting in and out.
The final reason or bonus reason not to is that say you get some contaminating and it's only your second pin, now it's very likely that all the rest of those loaded syringes are contaminated as well and you just wasted all that gear