Imo very few terraria mods add things in a balanced manner. And I don't mean just weapon balance (although every mod wants you to use new weapons, so powercreep is present at every stage of the game), almost all of them focus just on fighting, and everything else is sort of forgotten. The most popular mod, calamity, just straight up gives you mid pre hardmode accessories baked into the player, mining speed that is comparable to early hadmode pickaxes, and also just straight up rework harmode ore spawning. Many of these changes discourage exploration, because why look for better ores since you already mine fast enough (ore armours are kinda weak, even in vanilla) and flying island exploration can also be delayed until a better mount. Way too many non qol mods change fishing. And on my last modpack I got about 5 starter packs that gave me gear that had to be discarded.

And not just that, even if they try to balance things, they don't look at the whole picture. For example, on my summoner run I wanted a fire gauntlet (might be a wrong name), which is a fairly good accessory for summoner, a class that is fairly overlooked by calamity devs. But on the other hand, it is also a good accessory for the melee class, so they just pushed it way further into the progression, beyond moonlord iirc.

And as for the bosses added, they are mostly just reskins. Vast majority of mods adds just a worm, king slime, plantera or fishron AI and that's it. One thing that is often added and isn't as prominent in vanilla is a bullet hell boss, but I don't think this is the kind of game for that. Sure, not all bosses are bad (the ink witch from stars above is probably my favourite boss that's in terraria, despite being partly a bullet hell), but it's mostly just worms.

Another thing that comes to mind is accessory bloat. Terraria has a limited amount of accessory slots and vanilla already offers you multiple loadouts for each stage of progression. Boss drops can't be just weapons, so many mods just add a wall of text to each accessory (and often times weapons), while the end result is an equivalent to +50 health, but described as an energy shield that regenerates 1 hp per second if no damage was received in the last 5 seconds, max shield health is 10% of max player health.

In vanilla terraria if I have problems with a boss I usually stop trying for a while, I build a house, gather resources, build or arena or do something else, however it doesn't involve just fighting. While playing modded I usually just blitz through, since at every stage of the game there is surely a single weapon/accessory that makes the fight trivial.

Then there's also the entire category of qol mods. There's a great video on them from ilikecheese, that is pretty much how I feel about a good portion of them. The magic storage is needed for big modpacks, but I always trash the starter pack that just gives you the whole system.

Sorry if this answer was too long or not very coherent, it's kinda hard putting these thoughts together. Not all mods are bad, but with a few exceptions I think a currated boss rush would show off most of their content.