All disputes that can be the subject of a settlement agreement are eligible for mediation. Therefore, gambling debts, money laundering, tax fraud, etc. cannot be the subject of a mediation.
Other reasons why a particular dispute is not (yet) capable of being mediated relate to the intensity of the conflict or to emotional aspects.
A mediator will also refuse a mediation or, if it is already underway, terminate it, if they find that it is being misused to distract from its intended purpose: for example, a party is only sitting at the mediation table to extract information from the other party (fishing), a party only wants to start a mediation process to cause delay, ...