Is there a general stability result for the linear delay differential equation $x'(t)=Ax(t)+Bx(t-\tau)$ where $A$ and $B$ are $m\times m$ matrices? If not, is there a current summary of the known stability conditions? As of 2007, I believe the answer was no. Matsunaga had a nice summary in a 2007 paper titled, "Exact stability criteria for delay differential and difference equations" that specified:
I was unable to locate a more recent survey and didn't know if better criteria had been found since that publication.