My understanding is that TCAS is disabled not due to radar limitations, but because it isn't altitude-aware and will happily generate an avoidance command that results in CFIT.
(By the way, I believe EGPWS would take priority over TCAS anyway.)
One problem for sure is that when you're close to the ground you have to be careful about buildings, cell towers, etc. Terrain is one thing, but when you're a few hundred feet AGL, you could quickly be in the way of tall structures if a TCAS alert goes off.