You don't understand, the hemlock is literally the next best thing to useless. It's a giant gun strapped to an only slightly less giant engine. To fire its weapon, it has to divert power from so many secondary systems that it is skeleton crewed not because it Should be, but because the life support system is drained of power to the point that any more bodies on the ship would turn to corpses by the time they were done firing. 
The only reason it will work for Us is because it's 4/9ths engine, thus able to get us away from the battle in record time.

The plan is to hit them while the gun is firing, so the secondary systems will be off. We'll aim for so close to the bridge, they'll see us hit them through the window. After that, it's only a matter of fighting everyone who gets in our way. Plus, unlike orks, space marines, and admech, eldar are one of the few races that suffer quite a lot from poor atmospheric conditions. We'll be at the advantage while the life support comes back on fully, giving us the small window we'll need to kill everyone on the bridge.

Besides, you said it yourself; it's the Less insane plan. Given that we only have the two options, it's only sane to go with the less crazy one.