Its to do with 1 of 2 things. Spacer design, and suspension geometry change.
I'm sure you can understand spacer design issues - Incorrect centre bores and mounting surfaces can make for improperly centred and balanced wheels = vibration = bad. Slip-on spacers are often the worst in this regard, but poorly designed bolt-ons can be equally bad. The other problem with slip-ons is people not using long enough stud bolts, and then the added danger of using really long stud bolts without proper centre bore mounting on the spacer. This means that the only thing stopping the whole lot shifting is the tightness of the stud bolts, which are now longer, and subject to greater tensile, and shear loads. High tensile bolts are rated on their tension strength, not shear strength.
So basically anything more than 5-10mm should be a bolt-on spacer with proper centre bores to transfer the load correctly to the hub other wise you can get vibration, point loading, stud bolt shear and lose your wheels.
The second factor is suspension geometry which if not properly thought out will negatively affect handling in often surprising ways - especially with a lowered setup where the trailing arm is more likely to traverse past neutral. Especially in an e30 where we have semi-trailing arms and the camber changes toward negative as the suspension compresses. The more negative offset (created by larger spacers) makes this effect more pronounced (in terms of mm toe and camber - the angles of course are the same). Properly thought out, this can be used to your advantage, improperly dealt with this can cause interesting camber and toe changes mid corner and thus loss of traction or unpredictable handling. At the front there is a similar issue with castor and scrub radius - you may experience tramlining or loss of feel and wandering, and heavier steering.
This is not to mention the added load on the wheel bearings, hubs, springs, shocks and shock towers, the rear shock towers on e30s are particularly weak and susceptible to failure with high spring and damping rates and large negative offset.
Of course all this is at the extreme end, and the most likely scenario is simply faster wear, but you wanted to know why. You are wrong to think because you havn't heard any proof, the proof is not there. All you need to do is look and read - plenty of info on how offset affects a semi-trailing arm and McPherson strut suspension setup. All you have to do is look. Here for example
I'm not an expert, just a passionate enthusiast. I concede that 20-25mm spacers are no problem, and that 35mm spacers, can be used with some thought put into the spacer design, and the intended result. I guess what I'm saying is there is nothing wrong with doing it and there are also some benefits, but use decent spacers, and do the research first.
Why are you running such a large spacer anyway???
Oh and to answer the OP's question more clearly - the spacers pictured would probably be fine if you have stud-centering wheels and stud bolts. Otherwise the spacer must be hubcentric. I'd also want the spacer centre bore to fit over the vehicles hub snugly. (57.1mm for e30)