Machining is highly complex. As is the design of a turbine and it's housing. Tolerances alone on something that spins 100k RPM is insane. Not only does everything have to be located right. It has to be located to other features perfectly(what we call best fit). so you will say have a hole location, of x+1.375" y0 with a size tolerance of +.0005 a location tolerance of +/- .001 with a true position tolerance of .001", a best fit of .0005".Then you have dimensional tolerances of the part itself. Size, roundness, flatness, squareness, parallellness and many more. Let me tell you chap. On brand new million dollar equipment it's a nightmare hitting tolerances for turbine parts. I can assure you that Chinese factory is not using good equipment.
Now this thing needs balanced. Because no matter how perfect your cnc lathe may be, that part is still not balanced enough to see 150k rpm. Balancing equipment takes a lot of effort to use properly, and even more effort to keep calibrated. This i can assure you your cheaply made eBay turbo is not doing. Calibration is insanely expensive and needs to be done multiple times a year by certified personnel.
You then have different surface finishes that not only impact the look, but the strength of the part and the flow of air around it so what's better an surfsce finish of 125, or maybe 16? From my experience in working in the turbo machinery world when some of these things are capable of 150k rpm the surface finish is dead critical to it's material strength. I've seen parts with 2" walls peeled apart like pop cans from centrifugal force where the surface finish was far too rough near the start of a radius into a flange .
Let's go into material selection. Not all aluminum is the same. Cast aluminum will be different that billet aluminum. Then you have different grades of each of those. All with different properties. Are they using the right material? I can't answer that you need a mechanical engineer no of some sort that I'm sure they didn't use when they half assed copied a whatever turbo they are selling.
That's just the head of the problem. I can go on but I'm on a cell phone and won't.
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