What is minimum trace width for "advanced" fab shops and what are possible manufacturing/reliability issues?
If they are Sierra Ccts or "ProtoExpress", then they are a high quality, competent shop with adequate yields that always degrade if you do not have what it takes. 

Track and gap is only 1 of dozens of metrics for quality &amp; capability on fabrication.

Acid etching rate with not only sensitive to copper thickness but also temperature, fluid agitation rate, surface oxidation, density of nearby etching but also copper content dissolved in acid and more. 

It used to be 10 mil track/space was std then 3/3 for advanced but now it's;

Min   Trace and Space  (aka track and gap)
-----  ------------
4 mil  Std
3 mil  Advanced 
2 mil  Micro


Sierra Ccts. was my "goto" shop when I needed highest quality boards in the fastest delivery time and they never let me down.