A: 1 - Is this tool limited to CRM / contacts type applications?
When I look at the features, it seems like there is no such limitation, but I wasn't able to get a clear answer (asked you below one of your Youtube videos, ticketing system etc.)

You can create any Data Object Type as you need it to be. The Contact Object is the only default Data Object Type in Macanta.
We work on the basis that a Contact has a Relationship with a Data Object(s).

2 - Is "relationship" in your terminology, similar to the table relationships in SQL databases? I always see you showing contact related examples which leaves me in doubt.

Yes and No.
The back-end uses relational SQL DB, which is what you build when configuring the interface.
Right now we only have the following Relationship Options:

Contact(s) to Data Object(s)
Contact(s) to Contact(s)

We will be adding Data Object to Data Object Relationship options in the future.

3 - Is there a practical way to bind users to contacts when necessary? i.e. one-to-one relationship

Seeing as a User is also a Contact, you can use the Contact to Contact Relationship for this.

If you mean more as in a sales environment where an agent `owns` a lead, then you would connect both the Contact and User to an Opportunity Data Object. This would give the User and Contact an Indirect Relationship. You can then control access to contacts using the Contact View Permissions in the User management.

Hope that helps.