Hello,

.obj + .mtl are really obsolete formats.

.mtl is just a txt file that references textures path, and store some basic surface parameters (diffuse color, reflection amount), but it's not a 'universal' transfer format,
as every render engine use different way to describe materials. Most of the time you manage to recover the diffuse textures, nothing more.

PBR surface description is already easier to translate between render softwares (offline or realtime), but still require some adjustment.
Anyway, it''s much more practical to create materials in the render engine of your choice.