Not sure if its not working or I dont know what im doing. I click with a "break block" spell on it which is just Break Block on a vector constructed from coordinates (number constants). The spell works in my casting assistant device but just drains energy from the unit.

Version 1.16-2.7

EDIT: other tricks I tried work. This specific one doesnt work and doenst output any error.

Magic Processing Units can only break blocks that can be broken without a tool.

Got it. It should really say it somewhere in the wiki or wherever

Edit: or output an error when it can't break a block

I will update the wiki entry for Magic Processing Units with this information.

As for not throwing an error when a block can not be broken, that is just how the piece works.
I do not want to introduce MPU specific hidden changes to pieces
or make breaking changes that affect spells not using any Phi features themselves,
but you could submit a feature request on the Psi issue tracker regarding this.