Having a process like the ECJ or EFTA (where governments breaking international agreements can go through a tribunal process) is one thing. Giving corporations the ability to sue governments for laws that are "discriminatory" is another thing entirely.
In Australia we have a concrete example of what negative effects such a court would have. When we introduced plain packaging laws, many tobacco companies have attempted to use international trade agreement arbitration systems to overturn the law. They first tried persuading several governments (Ukraine, Cuba, Honduras and the Dominican Republic) to use the WTO court system to start legal proceedings against Australia[1]. When it looked like that attempt would fail, they tried to use the international arbitration system that was part of a trade agreement from 1939 that Australia has with Hong Kong[2]. This also failed because of a jurisdictional issue.
However if there was a court that was specifically designed to allow companies to sue governments for discrimination, Phillip Morris and similar tobacco companies would've had jurisdiction for such a claim (and likely they would've won, because the plain packaging laws explicitly discriminate against tobacco companies and were seen as very extreme at the time).
In fact, the only reason why the TPP explicitly excludes tobacco is because Australia complained and gave this same example. The response was just to remove tobacco from the agreement, not redesign the international tribunal system they wanted (which misses the point of the complaint). Unfortunately we've changed government since then, so we've now signed the new version of the TPP.
[1]: https://www.nytimes.com/2013/05/07/business/global/07iht-smo...
[2]: https://www.theguardian.com/australia-news/2015/dec/18/austr...