It uses 3 texture lookups, and need to sample world position, and blend the axes. It is more costy than regular uvs, but in a case where you would need fully normalized texel density and you need this on a lot of objects, it might be still a good idea. The one that can be used for non normalmap textures, is around 15 instructions, and the normalmap one is around 30. You can also disable "high quality normals" on the normalmap one. That should reduce its cost roughly to the half.
Though I probably wouldn't use it in a multi-layer material, at least not on all layers.
You can also use an object scale node. A bit cheaper than triplanar and doesn't get the weird skewing that can result sometimes with that method.
You can also make it local space,using various methods, so that "stretching", that is the result of how the axes gets created and blended won't happen when you rotate stuff around. But on like a spherical mesh, or one that has many surfaces facing multiple axes, would still have the artifact.
An easy way to manage texel density is to use a script. I use Advanced UV Normalizer. You can grab the texel density from one object (say a plane mapped to be 512x512 = 50 CM2), save that as a preset and then apply that to other objects. Saves me a lot of time and effort, not to mention it keeps my texel density consistent, at work.
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Though I probably wouldn't use it in a multi-layer material, at least not on all layers.
You can also make it local space,using various methods, so that "stretching", that is the result of how the axes gets created and blended won't happen when you rotate stuff around. But on like a spherical mesh, or one that has many surfaces facing multiple axes, would still have the artifact.