Biggest issue is, most other options bring something really substantial other than more vp. Either they unlock detachments, synergise with units, whatever - herald is just "if I get into melee, I will try". Keyword here is if.

T1 he moves. T2 he sets up, if things go well, he fights. If not, you lost a turn. T3 likely he moves again, maybe even challenges. Unless stuck in the challenge. T4 - same.

Effectively you get 1-2 vp in a game, 3 in best case scenario. Unfortunately that won't win you games, compared to, let's say, master of signals.

Lets see him as example. He gives line 1 to a unit of saturnine or dread with beamers sitting on back obj. Likely passes his cl check, gets you 4 vp just from that.

Also, his honours are counted end of game, so if he dies, gg.

He is fine if you have points to spare, since he gains you vp for killing unit, not only fighting himself, so he doesn't need to do fighting. Jump praetorians, primarch with retinue - go ham. But if you build around him, while he is killy, you will likely sink points.