I read on a couple of articles that Coffee Lake CPUs are not compatible with older chipsets (Skylake and Kaby Lake) because of different Intel MEI.
are there any facts confirming this? will it be possible to inject a Coffee Lake ME into an older motherboard?

Different MEI is the least of the problems.

Different pinout for the chips means that you can't get enough power to a Coffee Lake chip even if the older chipsets had a microcode/UEFI update.

I’ve heard also that... and makes sense for the new hexa core models.
An i3 8100 on the other hand has the same tdp and core count of an i5 7500. I see no reason a coffee lake i3 won’t work with kaby or even sky lake power delivery.
Even during the good old lga775 times there was some back and forward compatibility and that was a time when chipsets were much more relevant than today.

Some images have been released showing how the pins have been allocated differently on Coffee Lake CPU's.

The power delivery is different, similar to what happened with LGA775 chispets when the Core 2 Duo came out and required VRD11.