Even against a target dummy something which make no effort to 'charge toward' the player crystal bullets still have more single target dps than luminite ones.
Defense isn't enough to discourage using crystal bullets just fire a few ichor arrows every 10 seconds an you've basically eliminated the defense of the mech bosses.
The only time where I really notice crystal bullets being screwed by defense is when trying to use an onyx blaster or megashark against duke fishron's 30 defense after applying ichor.
Something to note about lumitie bullets is that you need to beat the final boss to craft them while crystal bullets are obtainable in early hardmode, if a post moonlord bullet is a sidegrade to one you get way earlier in the game don't you think that means crystal bullets might be a bit too op?
I think Crystal Bullets are more of a popular choice because most other choices are without any real purpose.
Silver/Tungsten Bullets, Party Bullets High Velocity Bullets are basically just slightly better Musket Balls. Of course HVBs travel faster, but missing enemies is more often than not due to human error than a too slow bullet speed.
Golden Bullets allow you to spend money to maybe make money, but if used on anything that doesn't drop a particularly high number of coins, they will just lose you money. Since you could just make an AFK money farm or use an endless pouch that saves you the ammo cost completely instead of having to hope for additional coin drops they aren't really worth using for anything. If you could quickly switch bullets and deliver the killing blow with these, they could actually have an application.
Nano Bullets apply a debuff that 99% of all enemies are just straight up immune to. I mean, just look at the list on the debuff's page. It might as well be a list of all enemies an on top of that even includes some critters! Why would anyone use these if even a basic Zombie is immune to it? Its only benefit over Musket Balls is damage.
Venom and Cursed Bullets apply a dot that is ultimately way too weak to give you a substantial DPS boost. On single, tough targets, the difference in damage is miniscule because the vast majority of the damage will come from the direct hit and against groups of weaker enemies, the impact is also insubstantial because the enemies will die before the dot has dealt enough damage to make a difference. Needless to say, many enemies are just straight up immune to the debuff. Then you basically just have a single hit bullet that deals a tad more damage than a regular bullet.
If the debuff dealt damage that is based on the target's HP (with a cap of course), these could work more effectively, but that can make these debuffs very lethal to the player as well.
Ichor bullets' only benefit would be in conjunction with other bullets, which are not reasonably switchable during any battle that is dangerous enough to require such optimization. As such, they are also without purpose because you're better off crafting a Golden Shower to apply the debuff on the fly.
If you use them exclusively, they are, once again, just normal bullets with a bit of additional damage.
That's 10 bullet types out of 15 and all of them function almost the exact same way, save for having maybe slightly more or less damage or a debuff that ultimately amounts to essentially nothing in the grand scheme of things.
Meteor Shots are the first more unique bullet type you can acquire and the only non-standard bullet in pre-HM that is more than just a basic bullet. It's also useless in the vast majority of the weapons players will use due to them causing i-frames, which makes other bullets hitting at the same time completely miss. The most popular guns are the machine guns like the Minishark or the shotguns like the Boomstick, all of which cannot take advantage of these at all.
Chlorophyte bullets only stop you from missing, but many bosses are big enough to not need that at all. I only ever used these for the Moon Lord and nowhere else. And without a dedicated Chlorophyte farm, these are also quite expensive.
Exploding Bullets could be fun and useful against grouped enemies like in invasions since they are basically the bullet-equivalent of Hellfire Arrows, but unlike Hellfire Arrows, these bullets can damage the user, instantly turning otherwise great bullets straight into trash.
Luminite Bullets are great, but not very useful for progression, considering they are post Moon-Lord. They're basically irrelevant by the time you get them anyway.
That now only leaves Crystal Bullets to save the day for gunners as a cheap and useful type of ammo. Maybe they outshine other bullets with their capabilities, but I feel like that has more to do with the other bullets being too mundane and uninteresting than with the Crystal Bullets being too strong. Crystal Bullets don't stand out all that much when compared to what non-bullet-firing weapons like bows, launchers and unique weapons offer the player.
I don't see a need to
discourage the use of Crystal Bullets I see a need to
encourage the use of a lot of the other types of bullets, which too often are barely more than a slightly better standard musket ball.