Short version of the question:

Any experience on copper sealing rings causing real problems when used instead of original aluminium sealing rings?

Question background:

Several times, when doing emergency work on my cars, I was forced to use copper sealing rings as replacement for the original aluminium ones - cause aluminium sealing rings are not so easy to get in my part of the world, and delivery times for the original aluminium ones can be months.
Out of my experience, aluminium rings are usually used to seal connections where at least on one side you have a massive part (e.g. radiator, or engine block) made of alu.
I have heard bad theories regarding the so-called electro-chemical corrosion which may occur in places where incompatible metals (like alu and copper) come in contact with each other.
Is it a real danger for real-life old cars like mine (ML 55 AMG 2001 + Audi S6 Avant (C4))? Should I be worried about the connections where I had to replace alu rings with copper ones? Have you ever had cases where copper sealing rings, when used instead of original aluminium sealing rings, caused problems?

Many thanks in advance!