In an all nurgle army, yes, they are good.
In an army where everything is SNP, they have much needed mobility.
They can also use a standard and act as a mobile teleport beacon for any deepstriking units you have. This is a good use of them.
I know some people give the leader an ether blade, the instant death on 1 attack upgrade, the 2+ poison for one turn standard (which effects the etherblade), and then try to use them to hunt down MCs. Personally I don't like this, sure you might get luck and kill the opponent in 1 go. But if you are unlucky the the MC using st 10 attacks will ID your flies pretty fast. Use with prescience and gimoire for best effects.
On their own merits they are pretty decent. Tough with many wounds and not bad in cc. Just stay away from those str 10 weapons.
I wouldn't bother with any of the upgrades for general play, keep the basic and get more stuff in your army.
To me, the biggest downside to them is that nurgle heralds do not have access to a fly. All other daemon cavalry can be supported by their herald, but not nurgle.
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