The forge is a huge improvement over the old system.

In the old system, you were limited on playtime. You had about 30 minutes to an hour and a half each day you could play unless you wanted to spend money to play more.
Then you had to pay money to upgrade your gear... And to revive... And to go down elevators... etc.

As it stands, the forge introduced the rarity system to the game. We now have free revives (at the cost of health revives. RIP), free elevators, free gear upgrades... but now heating gear takes longer. The difference with the forge to the old system is that the forge requires you to actively play the game rather than waiting 22 hours for another expedition.

Now this isn't to say the forge is perfect. End-game forge is pretty unbalanced and requires the best gear to farm some of the hardest levels. It's an endless cycle because you need the best gear to farm these levels, but to get the best gear you need to farm these levels.
Removing the chance of failure (or just making it a 80% - 90% - 100% chance across the board) would be a huge step in the right direction. Add more bonuses to the higher crystals, make them more common. This would make the fire crystal grind more tolerable.

Make rarity drops less affected by difficulty. Use the elite difficulty rarity drops as the baseline for advanced, and make normal give us 10% less drops and elite give 10% more drops. None of this "on normal you get no fire crystals, on elite you get 21". Alternatively make the drop rates the same across the board but have normal difficulty drop 1 fire crystal per box, advanced drop 2, and elite drop 3.

Biggest problem with the forge is the items required to make it work. The system is great, but the gathering of fire crystals leaves a lot to be desired.

+1ing what Fangel said, I'll take the forge, even as is, over the old system any day of the week. Definitely needs improvement though.