We did a few play tests with them. Stalker Pattern Boltguns are not bad, load them up with kraken rounds and you have some very nice range with AP, when they get a little closer vengeance rounds are fantastic, it doesn't cost too many points either.
S4 will Wound most tanks on a 5+ so combined with the AP value you can strip Wound off them, but the frag cannon still puts out a good number of shots and two in a squad can be devastating to anything that gets close and again has decent range and strength, you really don't need to force the half range to kill stuff with them. I found that if you are that close any way, you use the alternate fire mode
I think batttlescribe has a quirk, it will let you equip them with a boltgun and SPBoltgun. If this is legit, then it actually makes up for the high point cost given the new rules.
Missile launchers in Deathwatch are too high a point cost and do too little damage, sure they can put out a tone of shots but its AP 0, so most of them end up doing nothing. Unless you are playing against Horde armies with low armour values, then they are awesome, but you are better off just adding a friending Smurf Dev squad point wise.
Cheap units do not offer much protection, 2 fragcannons in a squad is amazing in overwatch, it means you only get charged in desperation/ignorance. I've found running them with a fire soaking unit, is more effective, the Corvus Blackstar works well as it can put out a serious amount of shots, so it a high enough threat to take up some of the fire. At 1500 points you can fit in a knight, but it becomes the biggest threat on the table and gets shot by everything.
Still tinkering, but some of the wargear restrictions feels very weird considering these are meant to be the elite space marines.