they imply that their advice is for everyone, and I am not sure it is...
I think it was Greenspun, but it might have been Strunk and White, who taught me this secret: Don't equivocate in print. Don't write like this:
"Here's some advice, which may not apply in your particular case: Do X."
or this (which you're more likely to see in actual writing):
"I think X."
Better to write:
Readers tend to understand that your writing comes from you, that it reflects a certain perspective and is unlikely to be universal truth. It's usually unnecessary to hammer this point home at the cost of cluttering up your sentences with clunky disclaimers that are always the same.
When you write, say what you think, straight out.  Don't settle for dull accuracy. If people get mad because something you say is wrong or overgeneralized, ask forgiveness, not permission.
37signals' work gets read and cited all the time because it is well written. One of the things that makes it so well written is that it's not larded up with apologies, disclaimers, and caveats. They assume that their readers are smart enough to figure out, without being told, that 37signals writes from a 37signals perspective and that your mileage may vary.
 Interestingly, speaking can be a different game. There's more room for artful equivocation. You can sell things with body language. You can vary the pace between telling it short and straight and telling it long and rambling. Novel-length writing is also different. We're talking about essays, here.