Bombs are hard to use without cloakers because they require a massive amount of forethought about where to place them, patience, and (in case of Imp) a reaction force to destroy stunned enemies. But in high-tier play - so rare in current ZK scene - they are often devastating.
Reef, like all ZK drone units, lacks a role statement and is essentially - and has historically been - a bunch of crap taped together: a little bit of Scylla in dgun, a little bit of Shogun with the drones, a little bit of airpad with the pads, a little bit of noobtrap, here i am, a little bit of reef to sink your metal. The same goes for funnelweb. Drones need a redesign and reimplementation.
Lobster is underused only because lobsters lob. It is absolutely terrifying in correct situations, like Scallops jumping into your base straight out fo water on Ravaged. But it's hard to use yeah.
Djinn is expensive, and does few things which are not done by either Athena (stealth deployment) or air transports (rapid unit relocation). One thing that it does kind of well is retrieve damaged striders for repair, but that's clunky (requires many clicks: deploy lamp, tell strider to get to the lamp, wait) and not that fast anyway.
Sling is good at what it does, but literally every other artillery unit is better because Sling cannot kill mobiles and everything else (besides Impaler) can.