I actually think this mod is fine in a conventional playthrough. At least, until hardmode. Since I just had a casual-ish playthrough on normal using this and the RPG classes mod. I wasn't playing in an optimal way, I intentionally played like I would have had I first started Terraria, which is to say lots of digging and slow exploration, as opposed to running across the surface to get start up gear. It took me a full couple days to trigger hardmode. Progression-wise, it felt very good. Although right after hardmode, I got impatient and just maxed my character out. Which is why I dunno how it fairs post-hardmode.
The weapons get pretty darn broken.
Tremor mod has a triple shot Stynger (which is completely ridiculous on its own), but then I +10'd it and gave it the void element and I was able to face tank everything. Plantera goes down in about 5 or 10 seconds, just face tanking because I can't die due to healing to full on every attack..
Then I did the same thing with the Helfire (basically a strong Megashark from Calamity mod), and face tanked Moon Lord.
This was all, by the way, COMPLETELY built for pure DPS. All accessories with Lucky..
I LOVE this mod in the sense that it allows weapons that get obsolete to stay relevant. The Onyx Blaster is an awesome weapon and the mod allowed it to stay competitive with other options further into hard mode. However, I do not think it is conducive to a playthrough for someone who wants a challenge.
I was thinking some significant overhauls need to be done. Specifically..
When upgrading a weapon, the mod should lean towards giving weaker weapons a stronger boost, such that all weapons are around the same power level by end game. This is probably a completely unrealistic option however as it would require individually tweaking every single item in the game. I wouldn't use an algorithm to do it either because it isn't just about the damage.
Probably much easier to do would be to nerf the living crap out of the Void element. It is far and away the best element choice, and I haven't even tried any other element..
At least that way you can't facetank everything while built for pure damage.