Yeah, it honestly just looks like a mess of random spikes everywhere. Not very visually intimidating nor aesthetically pleasing.
I am not arguing that I'm correct here, but stating my opinion on why such a ship is scary.
I think it lacks the spookiness/creepiness of Nightmare vessels, but to me it obviously looks extremely devilish and sadistic. Yes, it's a "mess of random spikes everywhere", which forms a monster of a ship. The mere look of it doesn't send a chill on your spine, instead it'll pierce your throat.
Also, for my comment on Shivan ships being "abominalized creatures", means that they look like monster versions of living creatures.
Of course not all Shivan ships look like creatures, but they are simply too weird that they don't look frightening any longer.
For those who look like animals, let me give some examples.
The Sathanas looks like a four-clawed beetle. Notice its underside; it looks like it has a "covering", and legs underneath.
The Lucifer looks like a lobster.
The Astaroth looks like a bat.
Etc.
Hell there's been a thread about this already.