I bought 2.5 litres of Chinese 72mg/ml nic from Bluestar-eliquid back in March 2015 and have been mixing with that. Also bought 3 litres from Darkstar (which is American nic) in March and September 2016. Both are clear PG base, both tested well for nic concentration and I can't taste any difference in my DIY juices. No taste or harshness in either.
I'm not a chemical lab, I can only test whether the nic level is what it's supposed to be, so for me the proof's in the pudding. I don't buy Chinese premade juices so that's not how I would judge it.
I vape using Chinese atties (Steam Crave) and Chinese mods (Yihi SX Minis) and there's no quality problem there. Sure, you can buy cheap and shoddy gear from China too. I understand why you make your generalised comment about everything Chinese but in truth there's a range of quality as there is everywhere in the world. Ever used a British plumber?