Meteor.js integrates well inside a Mantra.js architecture, but does it have advantages since it slows the running of my requests. For instance a dummy request in GraphQL (get a random string from the server) runs in few ms using Apollo Client and 1 second and more using Mantra.js (exact same request wrapping Apollo stack).
It only depends what kind of project you want to work on.
Mantra.js is an architecture framework for Meteor.js, that means : it helps a developer to follow the creation and development of its modules corresponding to the Mantra framework. The developer is free to use any kind of front-end library he/she wants, any Data Query and Client caching he wants, but in my opinion, it doesn't really improve the quality of the code. Someone can organize a project with own rules and it can be maintainable easily.
So if your team is comfortable with your code structure you definitively don't need something like Mantra.js.