JS bundling saves page size in MBs but bundled js loaded synchronously. And the browser waits for js download completion.
But in JS Merge Magento merges only those js which are required synchronously. And browser only waits for merged JS which is much lighter than bundled one. And rest of the JS loaded asynchronously.
Somewhere I have seen it is better to load js asynchronously if using HTTP2. And Google also recommends deferring JS.