Never trust politicians (and the police...).I believe $EMH has...

Never trust politicians (and the police...).
I believe $EMH has absolutely no exception on future tax changes.

Of course you have a memorandum signed by a minister, does that mean something? Yes, if you want to go to International Court of Arbitration, lawyer up, make a few appeals, wait 5-10 years until a judge will declare you were right.

The government can screw you over, happens in Mongolia (uranium project stolen), happens in Russia (350% mineral tax increase from January), can happen in Czech Rep as well, especially with a populist government desperate for votes and money.

By the way the country now also almost doubles gambling sector tax aimed specifically against one company, it is sort of a revenge of the prime minister against a local billionaire (Babis vs Komarek). The same prime minister who used lithium as a political campaign.

The memorandum mentions nothing about any tax incentives or tax guarantees (https://www.europeanmet.com/wp-content/uploads/2017/10/20171002-EMH_MOU-Signed-with-Czech-Government.pdf). The good thing is that the project likely will not be completely blocked and there won't be bureaucratic obstacles and delays made (which could've been the case before ČEZ involvement). Also the new law hasn't been accepted yet, it is in second reading now.

In the end, I think even if the tax increased 10 times (which isn't the case), the biggest impact on project's financial side will be how fast it can get started and what amounts of the side products (cement clinker, tin, tungsten) can be produced.

Other projects in the region (Zinnwald, European Lithium in Austria) are probably in much worse situation. Perhaps a better project could be Keliber in Finland with a huge resource of lithium just 5-10 meters underground, an open-pit mine, in a mire / peatland, just dig around a bit and you have 1% Li2O. Now compare that with Cínovec (up to 400m with lower content, even if magnetic).