The search for alternatives to Si in the VLSI technology is based on experimental and theoretical work. Here, we consider only the latter and, looking at a few two-dimensional materials of current interest, we use them as examples to emphasize the difficulties faced by theorists in assessing their potential: Silicene and germanene are examples of materials whose properties suffer from the strong scattering of electrons with flexural acoustic phonons; phosphorene highlights how small inaccuracies of ab initio methods prevent a reliable assessment of transport properties; finally, we consider transition metal dichalcogenides to show how surrounding dielectrics (the substrate and/or a top-gate insulator) depress significantly the carrier mobility and the performance of devices that use these materials as channels.