D1 levels: If crudest means worst, I'd have to agree about Level 5. It isn't 'bad', but (75% of) it really takes the term 'corridor shooter' to the extreme most literal possible iteration. Yes there are entire corridor only areas in other levels but in those cases, it's fine in moderation.
About largely symmetric, Parallax was clearly no big fan of it, as I can think of only a few levels in D1, D2, or Vertigo that even might come close. About custom levels, I consider some degree of asymmetry is always a plus but it isn't a deal-breaker either - Entropy, Entropy 2, and Mandril for instance are all great missions despite each having only one or two asymmetric levels, and even those asymmetric levels sometimes had large symmetrical parts. Obsidian has lots of symmetric levels, too, aside from the beautiful asymmetry that dominates the entire Icicle Cluster. Thinking about Mandrill again, I'm wondering if Luke started to influence Solrazor, for Chasm was completely asymmetric (short & quick boss level aside), then Phobos had symmetry in the first, last, and part of the third level, while Mandrill is largely symmetric except in the fourth level. Not that it matters too much, since these two authors definitely knew how to make a good game.
On the other hand, other prominent campaign authors such as Kruel, DFW, and just about any designer who contributed to TEW (even Sirius and Darkhorse, who moved away from symmetry after Obsidian) never focused too much on symmetry except in particular areas or levels where it would clearly fit. And the missions are certainly no less interesting because of these approaches, in fact I'd envision it's more enjoyable to explore and traverse the levels the whole way through. So in short - I don't mind symmetry too much, but I do give a small extra nod to asymmetry as well.