I want to make sure I understand what you are saying. You are getting 4x4 inch boards with 100 components fabricated and assembled for $10 each at quantity 10 including the cost of the parts? I take it these are very inexpensive parts?
I can't use their service because they only mount parts from their store. I can't design my board with JLCPCB components. But that is an amazing price. You might say unbelievable.
Are these boards fully routed, tab routed or V-scored?
Fully routed. Though they have a number of panelization options.
Relatively cheap parts? sure. But my board uses FRAM, high speed opto, lots of lower speed optos, 74 series logic (7, mostly NXP and TI), relay driver. The parts are much cheaper than from US/EU distis. By the way, that per board price includes DHL shipping to the US.
Are the PCBs cheap crap? Well, the first test is that >400 customers seem to be pretty much satisfied. Some wildly so. I have had 1 return and 2 failures that I replaced for free. One of the failures was a classic tombstone issue. No others from that batch had the problem.
Being critical, yes, solder mask alignment isn't perfect. But, it was good enough so the reflow worked correctly in the vast majority of cases. Also, they often use a solder mask that doesn't stand up well to reworking. Not the end of the world, especially if you don't actually rework the boards.
The parts library is, indeed, limited. In 2 ways: not every part in the world is listed and a lot that are listed seem to never be in stock. The current chip shortage has had some impact - STM8 chips for example. And they list hundreds of Atmel chips but only a handful are actually stocked. But just looking at what they actually have, it is still pretty big. If you target your design for chips they seem to stock, you should have few problems. In my design, I use a couple of 74LVC2G17 Schmidt triggers. JLCPCB lists 4 (iirc) different ones including NXP, Diodes Inc and TI. One is often out of stock but not all. So, when I make a production run I often have to switch. I have never had to postpone a batch due to chip shortages.