Panels will not be consistent - they never are (and you'd be surprised at the variation). Rivets will tumble in feed tubes or get trapped in escapements. Vision systems will get confused by ambient light or reflections at certain times of day. Weld spatter will accumulate on tips, and frequent tip dressing will change weld parameters (timing, pressure, current etc). Adhesives will be effected by temperature, humidity. Batches of sealer might not be consistent. Weld timers will fail. Power supplies will fail. Flow drill screws will deform, go off centre and be a bitch to get right. Laser welder lens will accumulate weld crud. People will mis-load parts. PIA bits will come in below the required quality. Tools will get shimmed wrong. Robots will be badly programmed.
All of this stuff will take time to get right. It isn't impossible, but it is very difficult to get a line performing reliably. Tesla will be integrating a lot of new technologies, and at volumes that they are not used to, in an area with no serious history of car building, in a high wage state. I doubt they can get the people to manage the line and maintain this equipment.
Isn't this exactly what Musk and his employees are good at fixing? I have seen no reason to bet against Musk, Your complaint is the same as the others saying he cannot do something. You seem to have expertise in how a thing is done now, not in how well it could be done.
He might be late, but he won't fail. As it is this is the scheduled shortened by 6 months.
Tesla will almost certainly be using a combination of materials on the model 3; mild steels (cheap and cheerful, easy to press), aluminium (lightweight) and boron steels (strong). To join these materials you need a variety of techniques including resistance spot welding, aluminium RSW (altogether more difficult than steel because of the low melting point and high conductivity, and the fact that weld spatter sticks to the tip of the gun), riveting (for steel-Al joints), structural adhesives, flow drill screws, laser welds etc. All of these processes take serious time to calibrate and get right, and some are not all that robust (by high volume standards).
There are many quality loops with a vehicle launch. The tooling needs to be proven out. Tools get shimmed relentlessly to make up for panel variations - panel quality can vary from batch to batch within a press run. This is especially true for mixed material cars.
Believe me, these are not easy problems to solve and nothing I've seen and heard out of Tesla makes me think they've addressed any of these issues.