Assembly, fit and finish might be a problem as the Fremont factory ramps up. But that might be mitigated by it's minimalist interior and exterior design. Excluding the drivetrain, components come from established Tier 1 automotive suppliers. I'd expect failure rates in line with industry norms. Maybe better as initial production volumes are much less than required by the major OEMs. 

I think the simplicity and reliability of EV drivetrains has been well established by Tesla, Toyota, Nissan, GM,  Ford, BMW and Renault. Tesla won't have problems with cells and battery packs. They already churn out gigawatts of battery storage.  The inverter and charger are established designs with a good track record. The Model 3 is a lighter less powerful car so the new electric motor and transmission will not be subjected to the ludicrous stresses seen in their performance S & X.  That leaves the autopilot hardware and that is common to the S & X and proven reliable.