Ask HN: How seriously to take offer letters?

I have this innate skittish-ness around signing anything that extends to offer letters (especially at startups with options), and often get caught kind of off-guard when places don't want to give you a copy of their stock plan even when the thing you're signing references it.

That said, I'm not sure how much it really matters? It's at-will employment, so either side can back out whenever, they retain the right to change salary, benefits, etc. Comp negotiations have already happened. It feels like it doesn't actually matter too much if I see these documents now or later.

If I find out they were misleading about something when I do see the docs a few weeks in, all that's really changed is that I've lost a few weeks of my time (and I obviously wouldn't be going forward if I wasn't banking on trusting the company I'm joining to begin with).

Things like non-competes and not-solicits are the only things that jump out at me as having a meaningful effect even in a situation where you join and quickly leave if you find out you were misled.

So given all that, part of me wants to just not make things too difficult and go forward. Am I oversimplifying? Are there important things that you can only really deal with prior to signing that I'm missing?