Reading through the changeset[1], this isn't a minor "some styles out of line" update, they're removing a number of polyfills that were there to only serve IE11 that affect many of their core components. Without testing it out, I would expect most components that use Javascript for functional enhancements to be broken. Most of the style changes are fairly minor and nothing you couldn't fix as you go, but the script changes are breaking changes.
[1] https://github.com/twbs/bootstrap/pull/30377/files