Of course, the science is definitely...odd, to put it mildly. You can just about swallow the idea of planets forming by all the rocky bits being attracted to pieces with more mass, thus causing more gravitational pull. Almost. A bit. However, the fact that planets form on the edge of the cosmos when matter spews out of the nothing beyond the edge is rather harder to swallow. The whole thing's like this too, but you quickly reach a point where you just have to suspend all disbelief and let it wash past you, so it seems unlikely that that's the reason so many people are cool toward this story.
It could be all the CSO that starts in episode two, but honestly, it's not really that bad (even allowing for the fact that the DVD has been cleaned up by the Restoration Team to lessen the fringing). The main problems with it are that it's rather apparent that they're not really in cave sets, as the lighting's not quite right between the actors and the CSO backgrounds (although, to be fair, this is a problem that still crops up from time to time even today -- yesterday's episode "Deep Breath" having a couple examples of this), and that they only seem to have a handful of backgrounds to drop in, which occasionally makes it difficult to work out the geography of the place. But that's about it, and if you can accept the other occasional questionable effects on Doctor Who then surely you can accept that?
Of course, none of this is to say that the episode is perfect. There's a brilliant idea at the beginning of part one, where the Doctor reveals that the Minyans (the people crewing the ship the TARDIS has landed in) were affected by the arrival of the Time Lords. "Well, the Minyans thought of [the Time Lords] as gods, you see, which was all very flattering and we were new at space-time explorations, so we thought we could help. We gave them medical and scientific aid, better communications, better weapons," the Doctor says. "What happened?" Leela asks. "Kicked us out at gunpoint," the Doctor replies. "Then they went to war with each other, learnt how to split the atom, discovered the toothbrush and finally split the planet." That's such a good idea that they should have made the whole story about that, but instead it's used to give the Minyans some regeneration technology so that they can have plausibly lived for a hundred thousand years. And this might be fine, except James Maxwell, as the ship's captain Jackson, has to tell the Doctor about how long they've been searching, and while it's intended to sound rather futile, Jackson sounds so intensely bored that it's hard to be interested in his story. And, sadly, this seems to be the speed that all the guest characters are operating at -- with the exception of Alan Lake as Herrick, who's just about the best thing on display in Underworld thus far. Well, that and the Doctor telling K-9 to shut up. Not that I mind K-9 that much (despite his initial introduction in The Invisible Enemy), but it's an entertaining moment.
But come on, being a bit dull and squandering some potential aren't the worst of sins. I suppose Underworld could take a hard turn for the worse in parts three and four, but as of right now its greatest problem is it's not as exciting as it needs to be. And if that's the biggest problem you've got (and we're still leagues ahead of The Web Planet here), seems to me you're doing okay.