One of my biggest gripes with the RBY series is how the more open-world environment affects the difficulty and your progression in the game.
The game starts off pretty straightforward up to Celadon City, where you get your 4th Badge, but after that is when the world starts opening up with multiple routes to get to other areas. Now, the open world idea is great on paper, but as a result of that structure, you have to account for players travelling off the beaten path, which often results in having level 40 Pokemon in areas where the toughest Route trainer has only level 30 Pokemon, so if you get any ideas about training Pokemon after your 6th Gym, be prepared to go through grinding Hell. Now, it isn't so bad in the FRLG remakes with the introduction of the Vs. Seeker and the Sevii islands as a way to train Pokemon at other varying levels, but as far as the old games, unless you had plenty of time, you were stuck with the Pokemon you had before the 4th Gym.