"As Matthew said, it is very stiff and hard. That means it runs poorly through a machine needle. This can create a lot of friction, which heats up the needle and that then melts the Kevlar in the needle. Messy."
Perhaps you mean Spectra, which has a low melting point.
Kevlar, like Nomex, chars rather than melts, which is
why it is used in fire fighting gear.
Kevlar behaves like a cross between thread and cable. It will
break down some when bent sharply-- like wire or cable. So
you lose some of the strength just running it through the sewing machine.
It also deteriorates in sunlight.
The strongest seams I know in Tyvek come from a heat sealing