Hive Guard
Hive Guard are presumably related to Tyrant Guard, as they look somewhat similar and have similar names. The Hive Guard are shooty, and quite powerfully so; they are equipped with what essentially are biological railguns that function as an S8 AP4 Storm Bolter that ignores cover and line of sight. As such, they're quite good at popping enemy transports, and can be used against most vehicles, if you can get them into the rear armor arc. With T6, 4+ saves, and 2 wounds they're also reasonably durable, having a toughness value too high to be instagibbed by high strength weapons, an okay armor save (though one that's ignored depressingly frequently, especially in a meta that loves it's mid-strength shooting), and a good toughness value.
They are the fucking bane of Rhinos of Fury.
With the sheer number of hive guard models Tyranid players bought in 5e to remain competetive in a world filled with mech lists, everyone saw the Nerf coming. It didn't matter to GW that the shifts in the meta and the rebirth of psychic choir lists had already shoved the Hive Guard to the wayside in 6e, GW saw something everyone already had tons of and decided to carnifex them (while ironically giving carnifexes some of the best boosts they've had since 4e) even as they came out with a new hive guard model more in line with the Tyrant Guard redesign.
The impaler cannon standby? More expensive now. That BS4 that made the Hive Guard so much more reliable? Dropped down to BS3. The basic gun? Now a really funky haywire gun that people are still unsure about, mainly because the Hive Guard has to get really close to use it. The impaler cannon now just flat out ignores cover outright, but didn't get the ability to have skyfire that everyone expected/desperately wanted, and so the Tyranids remain one of only two armies in the game with zero ground based anti-air from their army list (even including forgeworld).
These days, the Hive Guard is much less common, and much of it's competition has suffered as well. The Doom and Ymgarl genestealers are outright gone, the Deathleaper went from an iffy Elites unit to an iffy HQ unit, Lictors and Pyrovores remain bad, Zoanthropes got hit with a warp lance nerf, brotherhood of psykers, and both they and Pyrovores got smacked with the loss of spore pods. The Haruspex is considered "meh" but the Venomthrope has become the new crutch unit as you desperately need shrouded to close with the gunlines that make up the game today and you can't get FNP reliably enough to make psychic choiring your way to victory viable.
They seem like they could be great against the Wave-serpents making everyone miserable this edition by spitting out ALL THE S6-& from across the board while being nearly invincible before swooping in at the last moment and dumping objective campers (while also being too cheap and very easily spammed as dedicated transports). After all, you have a weapon that can dump out a lot of haywire hits and thus not really care about the serpent shield's defensive mode, or you can ignore it's jink save with a weapon that can reliably damage it.
But you'll never get that close due to the Wave serpent's ridiculous range, firepower, and mobility and your lack of range and mobility (also there's the entire rest of the Eldar army and their Tau/Dark Eldar buddies to deal with), and you have to choose between either ignoring their cover save but having to deal with the serpent shield or using a weapon that doesn't care about it's pens being downgraded to glances but has a cover save to deal with. Both of which suck. Not to mention Eldar players know the potential threat that Hive Guard pose and are going to make damn sure they never live to fire on a serpent. Drop a Tyrannocyte with a full brood of these glorious bastards within firing range and get your rape on.
God Lord Helix Emperor
Damn
It.