I don't think they are so difficult to distinguish, especially when in flower.
M. sempervivi has very few radial spines and they soon drop off, leaving just 1 or 2 central spines. The flowers also are quite definitely yellow.
M. formosa and its various forms, always have persistent radial spines, usually 2 or more central spines, and pale pink to white flowers. The only plant that differs in M. pseudocrucigera, often stated to be a form of sempervivi, but this has pink striped flowers, though its spines are more like the proper sempervivi. Pilbeam says that ssp. pseudocrucigera is the link between formosa and sempervivi - I agree in spine formation, but not in terms of flower, which is much darker than any formosa form I have seen.
I have heard, but not seen, of an apparently true sempervivi which has pink flowers, but in many cases the plant turns out to be pseudocrucigera.