Applying finder tags to applications - What are the consequences?
Since I've asked this a long time ago, now I have some thoughts about my very own question, maybe anyone else is wondering the same so here it goes.

Will I get an error? Will the app be updated successfully? Will the tag(s) remain?

Basically its a NO to both questions.

Why?

When you update an app it overwrites itself. So no tags, at least not as far as I know and/or tested. And tags will not break any update, again, as far as I know and/or tested.

IF someone posts a better answer I will choose the other answer over this.