Now I have another question: I wonder, which are the advantages or disadvantages of using the real scale materials over the traditional method?
Generally, it can be quite useful for archviz work, since it allows you to think of texture Repeat values as real meter sizes. However, whether it functions as intended depends on whether you generate UVs appropriate for its use, where appropriate means that UVs are generated using a 1m cubic projection, on the texture's Channel, for any objects using the texture.
The most important thing to understand about the feature is that it is simply an on/off switch in a texture, which communicates your intention for how that texture's Repeat values are to be interpreted. It cannot be more than this, because materials and textures exist independently from the objects to which they may be applied.
When Relative mode is enabled, the final real size of a texture tile is given by the formula 1 / Repeat * UV size. When Meters mode is enabled, it is given by the reciprocal, or simply Repeat * UV size. The point is that in either case, it is still necessarily relative, being dependent upon the real size of object UV space. For example, given an object with UVs generated using a 1m cubic projection, and a texture Repeat value of 2, you will obtain:
Relative: 1/2*1m = 0.5m real tile size
Meters: 2*1m = 2m real tile size
On the other hand, if the object UVs are generated using a 0.5m cubic projection, we see that Meters mode will not work as intended:
Relative: 1/2*0.5m = 0.25m real tile size
Meters: 2*0.5m = 1m real tile size
So the rule of thumb would be: if your host application natively provides the ability of generating UVs sized independently from objects (in some applications, UVs are always relative to object size), it will be best to use that functionality, generating cubic UVs of the desired size, and avoid the use of Meters mode in your Maxwell materials. In such cases, simply setting Repeat values to 1 and arriving at the desired texture tile sizes using the host application UV controls will be the best approach.
In other cases, it may be that your plugin will be able to detect the use of Meters mode and generate appropriate UVs on your behalf, but this depends on what the application SDK allows the plugin to do -- it would not do much good for the plugin to generate appropriate UVs, if they cannot also be reliably pre-visualized in the application, since you would then have no way of predicting or controlling how your textures are placed. Where this is not possible, it will also be best simply to avoid the use of Meters mode.
In general, then, I recommend avoiding its use altogether, and would personally prefer to see it removed from Maxwell entirely, since I consider it inelegant that the act of applying a material to an object (or changing a texture Repeat value in an already-applied material) should require alteration of the object's UVs. That may be convenient in certain circumstances, but overall, I think the feature has mostly served to produce substantial confusion, and would suggest that its function should reside either on the host application side, or in a dedicated object-centric tool, optionally provided by a plugin, where that is possible.