What's the Advantage of using MassEmailMessage instead of multiple SingleEmailMessage?

What MassEmailMessage appears to give you is a more optimal coding route if you are focusing on sending to Contacts, Leads or Users relating to either Contract, Case, Opportunity or Product objects.

So if you had 50 such Contacts to email you need only have a single MassEmailMessage instance and set a list of those Id's into it. Instead of creating 50 SingleEmailMessage instances each setup accordingly (you'll have to pull the email addresses yourself) to achieve the same. So there is arguably a marginal heap and statement benefit as well. As you say there are quite a few more features on the SingleEMailMessage, but if you don't need any of those perhaps the MassEmailMessage has a place.

If you scaling above my earlier example of 50 and working with these objects, this is indeed an additional benefit to those I've described above. Though ultimately its only dictating how many instances of each you pass in the array to the send email, which ultimately then governs overall what can be sent anyway.

setTargetObjectIds method of MassEmailMessage can accept an Array of ID for contact, lead and user id's, while you will need to create individual elements for the Array of SingleEmailMessage. However SignleEmailMessage allows you to pass any external email address.

So there is a Heap gain if you prefer to send emails to Salesforce Leads, Contacts, or users via the MassEmailMessage and is efficient.

However, if you are sending emails to external users both of these have a max limit of 5000 external emails per day per org.

Note that limits only apply if you are sending them to external emails. For Internal users, there is no limit and you can send unlimited emails.

You can monitor using the workbench REST utility via limit API and checking MASS EMAIL and SINGLE EMAIL limits.

The Mass email limits are described here

/services/data/v48.0/limits