That'll happen sometimes. When I drop an ironclad dreadnought to be scary and up front, I always pack hunter killers so it can have a better chance of killing a vehicle when it comes down. IIRC, furiosos have 13 armor, so it sounds like you just got a bit unlucky.
It really depends on who you're fighting and what vehicles they have. the same scarab swarms are probably gonna be useless against a leman russ line in the back and awesome against a dual land raider charge. I think the point most in the favor of the scarabs is that they are hard to kill quickly outside of assault, making them fairly low priority shooting targets, so they can reach the enemy. again, situational at best.
Well, maybe it is just 3 attacks, but let's assume a warsythe and a 24" move. thats 1.5 hits, and with S7 and 2d6 AP, that's almost certainly a penetrating hit unless it's armor 14, at which point you're still getting on the chart more than half the time. In general, it only takes 1 hit to stop a vehicle for a turn, and the tactical advantage gained from doing so, as well as the shifted target priority to this guy, is worth it IMO, allowing other units in your army a break from some fire. I use ironclad dreadnoughts in drop pods, which are also 190 points with hunter killer missiles, in the same fashion to good effect...and they're actually easier to take out when they arrive because of no cover save or symbiotic repair.That's still only three attacks - far from reliable even if it manages to get into position, and it's a lot of points. Even with symbiotic repair and all that, it won't be that difficult for the enemy to destroy it, I think. Why bother? It's unlikely to accomplish much, it's likely to cost you your leader and a ton of points, and you already have Scarabs and Wraiths for something similar - with the advantage of being cheaper, more generally useful and much more reliable at this. Most of the time, I'm quite sure the Overlord would do a lot more good bolstering some unit or another.