Comparison Table

Platform Meteor Horizon FireBase FireLoop
Description Open source platform for web, mobile and desktop. Real-time platform built on top of RethinkDB. BaaS Cloud platform for fast development of client Applications. Real-time platform built on top of LoopBack and Express.js.
Real-Time Yes Yes Yes Yes
UI Components Yes No No Not UI but WebRTC Components for Angular 2 are under development.
Client SDK Monolithic JavaScript Client SDKs for different languages Fully Typed Angular 2 SDK
Extensibility Meteor Modules Not yet implemented Plug-In mechanism None Isomorphic (Universal) Models, Hooks, Mixins and Components.
Scalability Uses MongoDB OpLog Uses RethinkDB Auto-scaling Adapter Driven Architecture (Options: MongoDB PubSub, Redis, Kafka, Build your own driver)
Maturity Very mature Not so mature, many pieces are not in place Very Mature Mature since it relies on LoopBack Maturity plus Modern Community Modules.
Open Source Yes Yes No Yes