No problem with eggshell for species that like alkaline potting mixes, but maybe would be with those that like the soil slightly on the acid side of neutral. However unless ground up finely I would think eggshell would break down too slowly to make a great deal of difference. Oysters shell in chicken grit may break down faster and so release its alkalinity quicker.
Really the only way you can be sure what PH cacti prefer in in habitat is a PH test around their roots and that may vary according when they have just been rained on with acid rain. The fact they grow on limestone does not mean to say their roots may not be in a humus layer above it which is slightly acidic. Far too few real long term scientific tests have been done on cacti living on limestone to see what conditions they prefer, meaning in cultivation you need a meaningful number of plants to do a test and a long enough time to be relevant.
One set of cacti in a slightly alkaline mix, another in a slightly acidic one and a third control sample in a soil of neutral PH. The problem is few of us have enough of the plants that grow on limestone to do such tests because often these are slower growing from seed and the rarer cacti . Also to be meaningful with our naturally slow growing plants the test may need to continue for several years.
One botanist even claimed cacti growing on limestone only grew when acid rain fell and the plants stopped growing when the alkaline bedrock turned the water alkaline. Certainly there have been trials of them growing well in acid mediums through acidifying the water but maybe that only means either the plants prefer an acid PH or even do not mind a range of PH's. Certainly most S. American cacti seem to prefer a soil slightly on the acid side, whilst most N. American cacti seem to tolerate one on the slightly alkaline side, but that does not mean they prefer it?
https://www.succulent-plant.com/soil_ph.html
https://www.rrm.me.uk/Cacti/Cactus%20an ... linity.pdf
Cacti being slow growing may often be forced grow on surfaces which quicker plants that may overtop and crowd them out cannot. Probably if we remove such competition from other plants in cultivation they can grow in many different soil formulations? However the PH of the soil can affect the availability of nutrients, particularly micronutrients.
http://www.cpm-magazine.co.uk/2018/03/0 ... rop-diets/
Some in fact prefer to mimic supposed alkaline habitat soils in order to slow down the growth of those limestone inhabiting cacti in order to stop them getting bloated and retain the hard slow growing habitat look.
https://xerophilia.ro/wp-content/upload ... Eaters.pdf
Gypsum has a neutral PH, unlike limestone for the plants growing on it.
https://www.pthorticulture.com/en/train ... ing-media/
, searching for some soil making tips. Some time ago I've found here some post about adding eggshell powder to the mix, it was said that it kind of making spines bigger or better, can't recall exactly. 
