Could somebody please PM me with suggestions for a US board house that does well with KICAD-produced build files? I assume that vendor recommendations are verboten in the general discussion. I need a 4 layer board with internal milled cutouts and I also need assembly from BOM if they do that.
thanks!
PS I am working with once now and they have been real slow and created a lot of what seem like persnickety DRC requirements.
I do not have a direct answer…but there has been plenty of mention of JLCPCB (and maybe a few other offshore fabs) on this forum so it does not seem that mention of fabricator names is particularly forbidden. I think it would be wise for vendors to do as JLCPCB has done, which is to recommend specific Kicad gerber output settings. I have ordered two 2-layer boards from them and found them easy to work with. No question that shipping time is a consideration.
Thanks BobZ. I deleted my earlier reply. Don’t want to say anything negative publicly about a vendor.
It depends some on your pricing expectations for a full turnkey PCB + assembly. I have successfully used two vendors multiple times, both quote full assemblies directly (and instantly) from the native Kicad files:
I’ve used PCB Unlimited before. I think also have a PCA side of the business, SMT Unlimited or something like that. Very similar to Advance Circuits, although I don’t think AC does internal milled cutouts.
Years ago I used Sunstone circuits/Screaming circuits for PCB and assembly. They definitely do 4 layer, not sure what their limitations are.
OSHPark does good work, but I can’t remember if they will do internal cutouts. They’ve got a few things that oval through holes that aren’t officially “supported” but I’ve had them on parts in the past and they’ve turned out fine.
For all of those vendors, I’ve generated gerbers using KiCad and haven’t had any troubles, with the exception of sending AC X2 attribute gerbers which they puked on several years ago.
There’s plenty of others, if you follow r/PrintedCircuitBoard this discussion comes up pretty frequently. 
I’ve used Bay Area Circuits before and as I recall the quality of the boards was decent. I don’t remember their pricing, but they do have an online quoting feature (as one would expect from any reputable manufacturer, and even some of the disreputable ones). They do have an assembly service, but I’ve never used it.
You can also check out PCB Shopper to get quotes on different board houses world-wide. Just ignore the ones outside the US if domestic manufacturing is a necessity (for example: government grant projects). Note, PCB Shopper won’t show you ALL POSSIBLE manufacturing houses, only the ones that PCB Shopper lists. I think only 4 of the places they list are US based. (Based on a quick and dirty check of prices for a 2"x3" board, BAC was the most expensive of the 4 listed US based board houses… Probably why I only used them once…)
Oh, I can’t speak for other board houses, but I do know that OSH Park does accept KiCad board files, AFAIK up to v5.1.x. I’m pretty sure they do internal cutouts (check their design guidelines for the minimum milling bit size), and they announced a little over a year ago that they now officially support plated slots (useful for some footprints).
I’ve successfully used OSHPark with a Kicad design.
I sent them gerbers from:
Application: KiCad
Version: (5.99.0-3436-g89dcc46898), release build
Yeah, if using the nightlies I wouldn’t expect any board house that is KiCad friendly to accept the board files. Luckily, all the KiCad friendly (and most KiCad naieve) board houses accept gerbers. 
I’ve always had great results just uploading my .kicad-pcb files directly to OSH Park; no need to create Gerbers. I think OSH Park can do everything you want except assembly.
If you want assembly, too, then you could check out MacroFab instead. I’ve done a couple of small boards with them, and they seemed good. However, they are very expensive. Also, be sure to follow their instructions for Gerber output, and in particular make sure your PTH and NPTH are merged into a single drill file. The first time, I sent them two drill files, and they fabricated my PTH but ignored my NPTH.
Both OSH Park and MacroFab are highly self-service, with little human intervention. (Although I’ve found that both have good support, if you need to ask them questions.)