Expect to see quite a few mistakes on assembly services. Assembly is far more difficult than just fabricating bare PCB from gerber images. There is so much to go wrong.
Also machine setup time is expensive, even in China, so ordering "a few" for test means hand placement of many/most of the parts. There will be human errors, it's difficult job to do right 8-10 hours a day on designs you have no idea about. Quite different to building a prototype of your own design where you "know" everything.
If you can find an assembler which almost never makes mistakes and is still affordable and quick enough, you are extremely lucky.