Twinax is a low cost 10G connectivity solution, which is very limited in distance (to achieve the required BER of the 10G standard).
Typically, it is used in UCS between IOM in the chassis and the FI.
If you need longer distances (>10+-m), your choice is fibre, which requires a SFP+ on either side of the link; cost is factors higher than twinax.
Northbound of FI one would typically not use FEX, which would increase oversubscription; 10G wirerate is best practise.
If you need to attach a lot of rack mount servers to FI, FEX might be a solution;