You get what you pay for & mass produced stuff usually tastes mass produced. Personally, I'm not a fan of any of it (if you are referring to Dekang, Hangsen, etc.) because it all tastes chemically to me. If I had to guess, I'd say it's like you said, they're using inferior ingredients. I compare it to the difference in a burger from McDonalds versus a burger off the grill in your own backyard. McDonald's burger may taste ok & fill your belly but, in an effort to maximize profit, it's made using inferior ingredients whereas the homemade burger is a whole different ballgame & not even close to being comparable.
I hear different opinions when it comes to shelf life. Some people say it's good for 2 years & some people say they are still vaping stuff they bought in 2008 & so I just don't know.
I definitely feel for you that you don't have any local shops because I know paying shipping & playing the waiting game stinks but if you're wanting good juice then it's what you gotta do & it's definitely worth it.