The most impactful issue with respect to sole vs. joint legal custody is having the legal power to allow the child to be legally adopted. A parent with sole legal custody can allow their child to be adopted without the consent of the other parent. This can deprive the non-custodial parent of any say in how their child is raised.
If the non-custodial parent has joint legal custody then their consent is required for the adoption.
This issue typically comes up in a divorce context when one parent remarries and the newly married step-parent wants full legal authority over their step-child.
I do not see the adoption topic come up in initial divorce mediations, but it does come up in follow-up mediations. There are ramifications regarding the non-custodial parent’s obligations for child support that can come into play and so a mediated agreement is often preferable than going to Court.