I also noticed that with a Combo a high level Card doesn't retain it's values, the Combo is somehow "fixed" to specified values and sometimes these values are worse than the OG... is that so? Why is worth upgrading and/or reasearching combos?
It's actually not fixed like that. It's just that the "Combo" tab is wrong. All the previews on that tab assume that both cards at at level 1. But when you actually use the combo in battle, it take the levels of both cards into account, and you will wind up with higher values than what is shown on the "Combo" tab.