@Eskape2play, you should limit the number of fight attempts for every fighter to 5, so we can't beat the same fighter to a pulp forever for extra money.
It is actually limited - if you fail to progress in ranking 3 fights in a row you lose and need to make another try (with some new bonuses ;) ). Else it is quite OK to go and grind weaker fighter after every new progression - it is up to you to set your own goals how you want to beat the game.
" I suggest “training" that covers when to take certain actions, what to look for, when to wait, the card system, upgrading cards, upgrading attributes etc. "
This one sounds cool!
"Had the status "you failed" after 8 or 9 fights in the pub. Why? "
It happens if you fail to progress in ranking three fights in a row, no matter win or loss. Not communicated clearly enough though.