I find the Merc to be excellent. It has really nice steering geometry - it takes some time to learn its character.
I don't want to be that guy, but you guys complaining - I think you're making one of two driving errors (or both). Either - you're not trail braking properly and/or you're steering in too much/too harshly on entry. Basically you're asking too much from the front tyres without first transferring weight onto them. The steering geometry of the Merc is just highlighting this error - which is probably intended as in real life your tyres might last only 15 laps on a good day if you drive like that.
With a ffb wheel capable of 4nm like the T300, it's going to be very easy to push the car past the optimum slip angle, and so the weaker the ffb wheel, the more likely you are too turn into that 'light' understeering state. The Merc is just good at showing you what you're doing wrong.
If you're rushing past 90degrees of steering lock on entry, you're doing it wrong. If you are dropping off the brakes completely and then expecting the cars front end to stick, you're doing it wrong.
I promise you that, if you just be more gentle with the steering on entry, whilst concerntrating on trail braking and allowing the tyres to progressively get up to tempreture, the awesome handling will reveal itself to you.
Every complain about this car seems to come from someone who has read some criticism, jumped in and thrashed it on cold tyes and said 'yeh it's definetly broken it understeers'
Just calm down and drive it like you're paying for the tyres.
These two tests are done with a OSW30nm with 75% muiltiplier.