In this article, I'll explain the differences between various mods in Project Zomboid, focusing on their likelihood of being buggy and other quality aspects.
(Russian)

Judging mods and modders is not a straightforward task. While I sometimes feel presumptuous in making these assessments, I believe discussing quality is crucial for enhancing the mod culture and habits in the Project Zomboid community in general.

The primary rule to remember is that the presence of bugs in a mod is more critical than the idea and game design of the mod.

These mods are yet to be fully explored and detailed information about them is currently unavailable.

Legacy Mods replace entire game files. This replacement is effective only while the mod is installed and active. Disabling the mod will reverse its effects. To identify such mods, look for file names in the mod folder that exactly match those in the game folder.

The main issue with these mods is the high likelihood of conflicts with future game updates. When developers release a new version of PZ, these mods may continue to replace files with outdated versions, potentially causing in-game conflicts. In some cases, they may even nullify new features until the mod is disabled. Mod authors must update these mods with each game update to prevent these issues.

It's worth noting that file replacement is sometimes unavoidable, as in the case of reskin or retexture mods. However, if a mod replaces the entire texture pack, it is still considered problematic under this tier.

These mods do not replace entire game files but may replace whole function or object definitions. The replacements are only active while the mod is enabled.

Identifying these mods can be challenging. One method is to inspect the mod's files, especially the item definition text files. If you find an item ID that matches one in the game's folder (/media/scripts/), it's a sign that the mod falls into this category:

The chance of future incompatibilities with the game and other mods is medium, but it's generally less severe than replacing entire files.

(i.e. Tier 3 or higher)

To see that a mod isn't legacy, just make sure it doesn't replace files, functions, or objects. You should have at least a little knowledge of programming.

Examples:

The creators of these mods are competent programmers or artists but may have approached their work casually, leading to typos and minor inaccuracies. These could cause issues, although the chances are relatively low.

Mod authors in this tier often rely on community feedback for improvements and may not proactively update their mods unless prompted.

To identify Work-in-Progress Mods, non-programmers should look at the description, icon, and screenshots in the Steam workshop. Detailed descriptions and visuals often indicate more effort put into the mod.

For those familiar with programming, reviewing the mod's code, especially Lua files, can provide insights into its tidiness:
Accurate modification of item properties looks like this (using Lua, not txt files):
OR (very accurate):

Starting with this tier, mods are generally compatible with most future game updates and other mods. They may remain functional even if abandoned, so the mod's last update date isn't always a critical factor.

Specialized Mods are bug-free, particularly the simpler ones. However, their game design might be unconventional or niche.

Assessing these mods often requires a subjective approach and might be best done by an experienced game designer. Some factors to consider include:

• whether the mod makes the game easier or harder,
  
• the level of detail in its mechanics,
  
• its balance compared to other game elements,
  
• its realism (after all, this is Project Zomboid!).

These mods do not exhibit the issues mentioned in the previous tiers. They represent a well-rounded and high-quality modding experience.

It's important to note that even Polished Mods can be affected by game updates, and the occasional red popup doesn't necessarily reflect poor quality. Mistakes happen, and this applies to both professional modders and developers.

As for me personally, not all my mods are Polished.

* Minor replacement, so the mod can be rated as Specialized/Polished.