One of the units that always gets mentioned when people talk about what is good in the Tyranid codex is the Hive Guard. This is Mostly because they are a good unit. They are one of the better shooting units in the codex, and one of the few reliable anti tank unit the Tyranids have.
1.) Durability: These guys are high toughness (6) with 2 wounds, and a decent (4+) save. These guys can also hide, making them more durable to shooting.
2.) Anti-vehicle Shooting: They have a high strength weapon (S 8) with a decent rate of fire (2 shots each). In addition they do not need LOS to shoot at a target. They are also BS 4 so they are fairly accurate.
1.) Range of 24” means these guys have to move quite far to be truly effective.
2.) Anti-infantry shooting: While they can kill infantry, they have only a So-So AP (4 is not much better than 5), and with only 2 shots each at most a squad of these guys will kill 6 models.
3.) Close combat: While Tough these guys have no armor save ignoring attacks, poor initiative (2) and few attacks.
4.) Synapse: This is a weakness for most tyranids, while these guys have a decent LD score, you don’t want to rely on them if they are out of synapse.
5.) Mobility: They can only move 6” during movement, (+ run if need be).
The biggest issue I find with these guys is the synapse rule. This means that I cannot always deploy the Hive Guard where I would like to optimally because risking them not doing what I want them to his huge. This means that if your whole army is pushing forward the hive guard really need to go with them. While this is not entirely a bad thing, for me it seems to slow the army down if everything else wants to run, the hive guard either need to run (which means they are not shooting.) or chance falling behind.