A simple answer to a simple question:
Is adultery grounds for excommunication or other discipline ...? ... I'm
talking about openly unashamed sinful behaviour.
The answer is "Yes".
...and its called "disfellowshiping" instead of "excommunication".
By comments under your question and the obvious fact that this is a personal situation of yours that prompts you to ask, there are many circumstances which people run from that sort of discipline, and do seem to evade it. There are many circumstances where contending parties have different accusations and versions of a story. Obviously there is no divine revelation to reveal the true nature of what happened, so there is the obvious human limitations that keep things from getting solved.
With that in mind, many people successfully (seem to) evade discipline, but there is the simple fact that its not humans they are disobeying, or running from. Here is the more important thing:
Galatians 6:7 [Amplified Bible]:
7 Do not be deceived and deluded and misled; God will not allow
Himself to be sneered at (scorned, disdained, or mocked [a]by mere
pretensions or professions, or by His precepts being set aside.) [He
inevitably deludes himself who attempts to delude God.] For whatever a
man sows, that and [b]that only is what he will reap.
Compare to Proverbs 6:27-29:
28 Can one go upon hot coals and his feet not be burned?
29 So he who cohabits with his neighbor’s wife [will be tortured with
evil consequences and just retribution]; he who touches her shall not
be innocent or go unpunished.
Marriage is God's arrangement, and typically judged by God himself. Disfellowshipping is the least of one's fears when they are blatant and unrepentant in their course.