Like you said, it's a multi-pronged and complicated test. I actually think Uber could meet a majority of the IRS factors.[1]
Namely:
1. Instructions: Uber does not instruct drivers about when, where, or how to drive.
2. Training: by most reports, Uber provides no training to drivers. (In fact, this is an argument which Uber detractors often use against it.)
3. Services Rendered Personally: as far as I know, Uber doesn't forbid you from hiring your friend to drive your car around with your Uber account open. (HIRING, SUPERVISING, AND PAYING ASSISTANTS might also apply here.)
4. Set Hours of Work: You can drive whenever you want.
5. Full Time Required: You can only drive a few hours a week.
6. Doing Work On Employer's Premise: This test is somewhat less applicable, but Uber doesn't require their drivers to (for example) come to a company HQ at the beginning of their shift.
7. Payment By Hour, Week, or Month: The bulk of driver payments depend on miles driven for customers, not hours "worked."
8. PAYMENT OF BUSINESS AND/OR TRAVELING EXPENSES: Uber does not compensate drivers for any of their (substantial) expenses.
9. FURNISHING OF TOOLS AND MATERIALS: Drivers provide all their own equipment.
10. SIGNIFICANT INVESTMENT: This is arguable, but many drivers are known to buy new cars just to drive for Uber.
11. REALIZATION OF PROFIT OR LOSS: This is definitely plausible. If drivers don't keep a firm grasp on their expenses or spend too much time driving around looking for riders, they can definitely suffer a loss.
12. WORKING FOR MORE THAN ONE FIRM AT A TIME: Uber drivers often simultaneously work for Lyft or other ridesharing services.
This isn't cut-and-dry and we shouldn't treat it as such.