The current behavior of et is that --rbe defaults to true, but then, if true, is ignored if RBE is not configured:

// If RBE config files aren't in the tree, then disable RBE.
final String rbeConfigPath = p.join(
  environment.engine.srcDir.path,
  'flutter',
  'build',
  'rbe',
);

This leads to really confusing behavior, where you think RBE is being used (and indeed the flag is set), but silently the CLI has internal heuristics that decide not to use it. For example, I thought it was enabled, but there was no output, and I was taxing my CPU pretty heavily:

Proposal: We need to stop using this dark pattern of:

void doFoo() {
  if (inscrutableGlobalState) {
    // Failed successfully.
    return;
  }
}

In this case, I'd recommend that:

// If RBE config files aren't in the tree, then disable RBE.
final String rbeConfigPath = p.join(
  environment.engine.srcDir.path,
  'flutter',
  'build',
  'rbe',
);

argParser.addFlag(
  rbeFlag,
  defaultsTo: io.Directory(rbeConfigPath).existsSync(),
  help: 'RBE is enabled by default when available',
);

Now, I can do:

et build --help

And see if RBE would defaulted to true, and if I run:

et build --rbe

... and RBE can't be used/found, it should result in exit code 1/failure.