Ash had an easy run through the Battle Frontier compared to the challenge of the games. He never had to battle dozens of Trainers to earn the right to challenge the Frontier Brains. He didn't have to adhere to the unique rules of the Frontier Brains' respective battle facilities. The anime's substitutes for these rules, if any, rarely offered a similar level of challenge. He also didn't have to challenge each Brain once for the Silver Symbol and again for the Gold Symbol; he automatically received the latter upon winning. Most importantly, he rarely had to fight a Frontier Brain with their in-game team, especially if they involved Legendary Pokémon.
Source：https://www.cbr.com/pokemon-ash-vs-peak-brandon/
