They are quite different, speaking of Bazel it’s designed to be an all inclusive build system for any language at essentially infinite scale. Which means it:

- Has nice Python based DSL

- Strongly encourages 100% explicit dependency specification

- Has built in caching (local and remote)

- Built in test result caching

- a fully featured build graph query language

- Built in distributed execution support (for any build step)

- All work scales by part of tree you are building not it’s absolute size

- Support for fetching external source code and binary deps

- Has first class support for executing and testing containers

- Has first class support for code gen (ie you can build a compiler use it to generate code, then build its output,and everything works, no hacks in one build system)

The downsides:

- Works best if you all your code is built with Bazel, which makes deps hard

- Support for Python is weak, Ruby non existent, and Node is beta quality

- Has essentially zero convention, everything must be explicitly configured (ie explicitly describing go deps)

- Has a memory hungry local java daemon

- Has a more overhead than ninja (but is more accurate because of hashing and isolation)