you have to understand that the real priority of motoring journalists is not to give true comparisons, but a need to satisfy their advertisers - who put far more money into their wretched magazines than is raised by actually selling them!
Most journalists are quite young, with limited driving experience and ability, which they compensate for by reading other people's reports, and add a mixture of their combined prejudices, gut feelings, and errors. Any car with more power than its chassis can handle is regarded as giving 'Driver involvement and satisfaction,' even if that means it wants to swap direction when it feels like it.
Another blind spot - because the writers never buy a car with their own money - is running costs, such as problems with the twin-screw supercharger. Assuming the minimum standard of servicing (a Full BMW/Merc Service History) it has probably never had an oil change, and that is the tip of the iceberg. A £4k 1.9 Z3 will be a good honest car, but a £4k Merc is great expense waiting to happen! Bits for a Z3 can usually be sourced from German & Swedish, but Merc parts are a different matter.
In this case, the comparison of a 1.9 Z3 to a 2.3 litre supercharged SLK tells you a lot . . .