I guess newbies in this space, see the prominence of JLC, and expect they must be good -- good enough to apparently be popular -- and look at the price and figure, how can I lose, it's so cheap?
The history as I understand it, is JLC is one of the newer entries to the market, and through "shady" dealings (spamming forums, sponsored promotions), and I presume, considerable automation on their backend (both for processing files, and in the actual printers), have cut prices lower than ever.
(Or was that PCBWAY that was spamming? I forget already.)
So some may be unaware that such services really are cut rate, for all that implies.
You get boards. They can be soldered. They probably don't have shorts/breaks differing from the fab files.
You get their stackup. (Custom stackup? Ain't no one got time to read that.)
You get their materials (whatever soldermask, silk, etc. ink are used).
You get their quality process (whatever range of blemishes, plugged/tented vias, soldermask webbing, plating thickness, layer alignment, etc. they consider acceptable).
If you want something slightly off the beaten path, think for a moment: should I be shopping for $thing at Walmart, or a store specializing in it?
Do you buy your tools at Walmart? Harbor Freight?
Mind, not setting up a guilt trip here: there is a time and place for everything. I have some HF tools. I don't expect much of them, and they get it done most the time. Simple cheap protos -- maybe you're going to rip into them anyway (because you goofed up on component choice, wiring, etc. -- yes, I've done this plenty of times, too!), and honestly, the weaker grade of soldermask makes it easier to expose traces, making new pads and such.
But if you expect something a bit tighter, or well supported (customer service), or a custom stackup, or production quality -- you're expecting too much from them. They simply aren't in that kind of business. Go elsewhere; you will pay more, and you won't regret it. You may even pay less, in quantity (100s, 1000s of units).
Likewise for their assembly service: don't expect rigorous AOI, let alone flying probe test or rework; placement and soldering defects have been reported here before. Expect some amount of fallout, or do the rework yourself.
For reference, typical full-service board fabs are in the $100 range, and assemblies in the $1000 range. Eye-watering for a hobbyist, true; but very much, cheap to on-par, for the professional.
Tim