I have always considered charcoal as a potential source of problems. Once it is loaded with whatever it absorbs and starts to release it, trouble is approaching. Then it is time to repot. Without charcoal, no accumulation and less urgent with repotting.
If you do a search on charcoal you'll find lots of comments.
I question this Bjorn. What kind of ''accumulation'' are you talking about? I don't know (can't find) the difference in the CHC between bark and charcoal but the point is that char does have a CEC. (activated char only higher) This means it will attract base cations like Ca, Mg, K, Na. It does not ''hold on'' to them and release them after some time for no apparent reason. The cations are exchanged for others as they get used by the plant or the concentration in the medium water changes. This is a good thing. It means we can reduce the amount of nutrients given to a plant and still get the same results.
We often see roots clinging to pieces of charcoal which means that it has the physical properties which plant roots find attractive and because of the CEC it also means that these are sites of nutrient absorption. As opposed to for example polystyrene. Roots also adhere to bark but because the surface of bark is slowly broken down by microbes, the connection with roots is more temporary and to my mind more of a source of problems than charcoal.
Once the char becomes loaded with cations, it is not possible that it will continue to accumulate more and more. It is limited to the amount of negatively charged sites. Those there are simply exchanged for others when the environment around them changes. Peat has a very high CEC (much higher than char or bark) yet there is no talk of peat having a problem with accumulation. If we are talking about plant ''wastes'' like dead cells, gasses or whatever, these materials are not held in charcoal any more than any other porous medium and flushing with water will remove it from the substrate.
I really don't see any problems with using charcoal but I do believe it has beneficial properties and I think this ''accumulation of salts'' issue being been as a problem does not appear to have any basis in fact.
In theory, we should be able to grow a plant in pure char for an indefinite period if managed correctly.
The so called ''sweetening'' effect of char due to it's high pH is I think over emphasized and probably only temporary.
There is a commercial orchid nursery just down the road from me. They use a 33/33/33 mix of char, bark and perlite (or lava) and would not dream of changing the mix.
I've been using charcoal for many years and I can't say I've ever noticed a problem. What is probably most important is using a good clean product which has not been contaminated with some toxic substance.