The Mercedes 722.6 is pretty bombproof, and what issues it can have are readily fixed on the drive (a lift obviously makes it easier).
BMWs shortcomings largely stem from making most of the engine ancillaries out of the least heat and oil resistant plastic they could find and a VAG approach to component design.
At least, being a Toyota, the Z4 should be pretty reliable
In order:
in the real world, the gearboxes lose all drive. The only realistic fix is a replacement gearbox, and doing that on the drive is not for the fainthearted. Assuming you could even find one, as demand was high which meant the prices were too.
I wouldn't call the timing chain an ancillary. But you are right about the plastics, which also affect Porsche.
Do you really think that BMW, who recently used
The value of perfect engineering as a slogan, feel humble enough to learn from Toyota whose speciality is mass-producing boringly reliable stuff that's also affordable throughout its life. Although they did make the Supra even uglier than the Z4, which is quite an achievement