One of the big concerns from a technical standpoint is the way that the materials are setup on Reforged. If you imagine a model is made of many small triangles in 3d space, and each triangle has a 2D image file painted across it to add detail, at this fundamental level Reforged models have new code to allow them to look sharper with the same number of triangles.
The idea is that you have a "Diffuse" texture, a "Normal" texture, an "ORM texture", and an "Emissive" texture. Each file can be thought of as a pixel grid with 4 numbers each between 0 and 255. So, in the old War3 models and the earliest WoW models, none of this existed and there was only one texture which is what the new Reforged way of handling things calls "diffuse". This is not specific to Reforged so you can most likely find out more about it online with some basic searches.
But what this means is that when the computer resolves a single pixel on your computer screen based on:
1) A triangle that this 2d pixel falls inside of when we draw a ray inward in 3d space from the screen to the model
2.) What to draw on your 2d screen on that pixel
... it is now the case that the graphics card per-pixel computer program now loads Red, Green, Blue, Alpha, Normal X, Normal Y, Normal Z, Occlusion, Roughness, Metalness, Team-color-ness, Emissive Red, Emissive Green, Emissive Blue, and (maybe) Emissive Alpha. (I forget whether emissive has alpha since solid black might mean the same as alpha, so maybe emissive alpha is not necessary.) Then it decides "what color is this pixel on the screen" based on about 16 numeric values instead of based on the 4 values old models have (the 4 being Red, Green, Blue, Alpha).
Most of my experience has been with testing out old 2002 Warcraft 2 SD legacy models and what it is like to re encode them in the new Warcraft 3 Reforged format. So, in WoW, you might find models that have newer systems and more textures. Maybe they would have a Diffuse, Normal, and Emissive texture but not have an ORM texture.
The result is that there are mathematical reasons for why, but your WoW models are different in "noticeable" ways.
The problem is that, if we forget about the formatting and how your model textures are stored, when we go to draw a pixel of a Reforged model it loads 16 numeric values, but if we load your ported WoW model then probably at least 8 of the numeric values are going to be bogus or assigned to 0 or something.
So you need a smart tool or an artist to introduce in this new information on the WoW model. What is the metalness of each pixel? What is the roughness of each pixel? What is the ambient occlusion value for each pixel? In addition, a lot of people who port WoW models would port them to the 2002 model format, which also uses the ".mdx" extension, and in this format it is physically impossible to specify all 16 values by specifying the material made of multiple textures for different purposes. So you get a model that is not only different quality, but basically cant be fixed until you change the format to Reforged format.
Edit: My knowledge comes to me as one who modded Warcraft 3, not other games, so there are gaps in my knowledge and I am not the most knowledgeable about these topics. I am told that Reforged uses what is called PBR, or Physics Based Rendering. If you research this online, there might be more about how to convert something to look good in this system.
