In regards to dimensional accuracy, that is a very tricky one with FDM. Much like surface finish, this will vary from printer to printer depending primarily on the nozzle size.
The bigger issue, I think, is that for a good final print tolerances need to be built into the design of the model from the start and most of the time, that does not happen. The reality of 3D printing (or any manufacturing technology) is if you have a hole that needs to accommodate a 2" diameter pipe, that hole can not be 2" in diameter as well. The printer will never exactly replicate the model, and it’s unrealistic to expect that. These are core concepts of mechanical design, but not something your average customer will understand. Even if they did build tolerance into a model, the right tolerance for one printer does not guarantee it’s the right tolerance for another. I know with mine ±0.4 mm is a realistic tolerance for loose fit and I can push ±0.2 mm for snap fits and gears, but that’s only for me. I could buy a new printer and it behaves very differently. This is also why I make it explicitly clear to my customers that I will not guarantee the final part fits dimensions they specify unless I design the part, I only guarantee it will match the model to the best of my printer’s ability.
I think unfortunately, the reality is you will never get true “dimensional” accuracy from a setup like 3D Hubs, unless the specific Hub models the part. There are just too many factors that are not controllable at play here and it’s not like a standard prototyping shop where you get toleranced drawings to work from. A good Hub can definitely get it close to desired if they work with/can get information the customer and the Hub is willing to correct models/the customer is willing to pay for remodeling, if necessary. I do not think it is fair to expect a Hub to print a pre-made model to exactly fit unknown requirements (especially from an STL, not a solid model), the only thing that should be expected is the print matches the uploaded model as closely as physically possible. I also do not think it is right for 3D Hubs to promise “dimensional accuracy” to the customer, when there is really no way to guarantee that.
It’s like if I order some graphic to be printed on a t-shirt from a silk screening company; it is not the shop’s responsibility to ensure that spelling, design colors, and alignment is correct. Their only responsibility is to replicate the graphic as best as they can on the t-shirt in a location range that I specify. It is on me, the customer, to make sure the design is correct in the first place.
The other reality is most customers here are not trained in modeling/designing for manufacturing, nor do they have any interest in learning (obviously, this is not true of all customers). They just want it printed so it works how they want it to, which is not an unreasonable demand, but the understanding has to be there that this is not a perfect process with even the best operator and printer, and there is a significant increase in scope of the order to make sure a part fits dimensions.