OK, you can't really do good statistics on a batch of 20 units. BUT I recently tested all the received boards I had manufactured from dirtypcbs.com for resistance and functionality, with interesting results ;)

Yield

5 of the 22 boards I received (20 were ordered) arrived defective. I contacted dirty for the issue, and they refunded me for 3 boards (since i ordered 20) within a day. The defects are kinda interesting, since they seem to be were similar, close to identical on all defective PCBs. Here are some:

Resistance variation

The rest of the boards was fine, and I could measure the resistance of them:

There resistance varies about 30 %, and even though this is a lot, for any normal PCB layout this wouldn't be an issue. Since the resistance of the traces simply defines the maximum power output of the heated bed, they're perfectly useable and a good proof of concept.

Small quantity prototyping

If I would have let the prototypes be made with a german board house, I would have paid about double the price, since even though the boards itself are about 4 times cheaper from dirty, the shipping of the large boards eats up most of it.

If I'd do this again, I'd probably use the german board house from the start. Even though I got a refund on the failing boards, shipping is not refunded, and somehow I lost my trust in their e-test ;) However, high-RPM prototyping services like dirty (or similar) chew through individual small quantity orders at an amazing rate and for an incredible price, they're still the best for their 5x5 protopack.

Manufacturing

For preproduction and final manufacturing I'll work with a high reputation PCB house from Germany I've had good experiences with in the past. I've already received quotes from them (and others), and given enough interest in the board, maybe this project will get eventually get there.