The thing that bothered me the most was not the battle frontier as I only really liked the factory and the tower. It was the fact that it was not an Emerald remake packaged as a R/S remake; not being able to take on the two villain teams, not having the whole trio integrated into the game (remember how terra and marine cave were dynamic?), removing the new locations Emerald added (desert underpass, for example), no gym leader rematches... It would've been incredible with all the QoL improvements. Battle Frontier was cool, but I can get enough of a battle fix from things like Battle Maison and Showdown. There are more pressing issues, for me personally.
