Apologies for the incoming novel. TL;DR: Iconoclasts, under the right circumstances, have a decent, but not great, chance of crippling a fresh battleship. Feeder tendrils don't do much damage, but simply destroy a ship's systems several times and limit repairs, thus de facto crippling them.
Even with Locked On Iconoclasts within 15 cm of a closing battleship, vs. a 5+ braced ship, you're looking at 7.5 damage. That's 1.25 crits, so you'll usually expect 1 critical effect and an average of like 0.57 damage from crits; let's call that 8 total. While 10 Iconoclasts can ruin a battleship, they often (that is, more times than not) won't even cripple it unless it's already on a few blast markers.
Let's compare that to 12 Drone escorts (240 points at 20 points each, so comparable to 10 iconoclasts at 25 points.) Armor's irrelevant, as are shields, so the only defense we need to look at is bracing. You're going to score an average of 18 hit-and-run attacks and 6 wounds; brace will reduce that to 9 and 3, respectively. 5/6 of your hit and runs will cripple a system, with 4/6 crippling a weapons system; on average, you're going to break 7.5 systems, with 6 of those being weapons. Those hit-and-runs have an expected damage value of 1/6, not counting Fire!, so you're going to pile on a little more than 4 wounds on average, in total. None of this is taking into account the 50/50 chance you have of critting off your up-front damage.
So now you've got a battleship that likely has 4/5 of its systems out, with 8 repair dice — but halved, since you've got a bunch of tyranids sitting on your base. (I haven't yet run into a situation where I didn't park my Drones on my target.) With 4 dice, you're going to see one-or-more 6s 52.8% of the time. You'll only see two-or-more 6s about 10% of the time. Your best-case realistic scenario is repairing a single weapon system and having two online next turn; the most common scenario is a 50/50 chance at having a single weapons system operational.
Keep in mind that we gave the Iconoclasts the best possible scenario: They could Lock On with a closing capital within 15 cm. The Drones will get their result from any angle, without orders. Also, you will see 12 escort Drones; they're tax ships. Nid player must take them to field Charybdis and Proteus ships. I'm not convinced you'll regularly see 10 Iconoclasts.
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I'm fairly new to BFG, so I'm not sure how much weight anyone should give my opinion, but my games with nids have been one-sided affairs. I point at the enemy, roll All Ahead Full once, and then begin deleting ships, usually on turn 2. The games are often decided in the first round of shooting: Did they kill enough of my escorts to limit my feeder tendril spam, or not? If they did, then I'm in for a bad time: Nid guns are ungainly (6+ lances and front-only batteries). If I get to feast on ships, then I'm basically halving their fleet points in one go. It's a very all-or-nothing fleet in its current incarnation, and that leads to frustrating, repetitive games.
Personally, I think that feeder tendrils only become problematic when you can load them up on cheap bodies. I don't think they should be available, period, at 20 points a piece; they feel fine on 35-point Kraken Predators, where they're strong but not overwhelming.
My solution: Don't offer feeder tendrils as an option on Drone escorts. Nid shooting in general might need to be slightly better to compensate for no feeder tendril spam (with my sense being that Bio-plasma could get a sprucing, perhaps limiting its shield-piercing to within 15 cm but making it a 4+ lance at all ranges), but I haven't played enough games w/ no-tendril Drones to really get a feel for it.
Edits: grammar