Currently, from my understanding…
In Dragon’s Descent, you have the chance of finding royal gems on the floor of the dungeon in batches of 1-5. Wild Pokemon can drop these as well, but they seem to have a fixed drop of only 1 gem, given that they drop any at all. The bosses, which you fight on floor 50, each have a CHANCE to drop 5 gems each. This means that, if all 5 do have drops, which is a very rare occurrence in and of itself, you will only obtain a maximum of 25 gems from the boss.
As for Dragon’s Descent Depths, the following dungeon which contains an extra 20 floors, the drop rates stay the same. After beating the boss, you do get a guaranteed(?) 20 royal gems. Additionally, you also have a chance of getting a certain amount from the two chests at the end, though this is never guaranteed. In total, after comparing totals from three different runs, I averaged around ~410 gems per run.
I believe that this is where the largest problem that I have with this lies, as this is a remarkably low number for the sheer amount of time that it can take to clear this 70 floor pair of dungeons. The goal for most people in running this dungeon is to gain enough royal gems to purchase the Coveted Key from Taric’s Shop, which costs a whopping 10,000 gems. This means that, realistically, it will take most people around ~25 runs of the dungeon in order to accrue enough royal gems to have the chance to get a Dratini.
I think that, in general, despite the wealth of royal gems that you can find in this dungeon, the fact that they are more often than not obtained alone or more rarely in a set of up to only 5, makes this process far more time consuming than I believe is necessary. Comparing this to obtaining Rupis in Dynamo Sands, you are able to find them in a much greater abundance that makes getting a Gible a far more realistic and achievable task. The fact that they are obtained in such miniscule numbers makes the process so much more excruciating in Dragon’s Descent, and that is exactly why I think this could be improved upon for people who might want a Dratini in the future but do not have nearly enough time to put forth for this unbelievable grind.