Solidify adds an unnecessary amount of extra triangles onto a model and using it a lot can lag your game. The “DoubleSided” feature on Roblox fixes the problem of some exports in Roblox Studio being transparent from certain angles, but if it has the same consequences of Solidify then, in my opinion, it’s not worth using.
This has nothing to do with the Solidify Modifier in Blender…
The DoubleSided feature is (in a way), related to the Geometric Face Orientation in Blender, through Normals. (as far as I know)
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As far as my understanding goes, it just disables Backface Culling and renders things more like how Blender does by default in the viewport.
Also, Solidify is not inherently a laggy and bad modifier, you just have to know when and where to use it. So while using it to bypass Backface Culling is impractical when DoubleSided exists, don’t hesitate to incorporate it into your modelling workflow instead.