we've been using a sharing network for about 7 years now and have shared a handful of needs now. we're quite happy with it.
the thing that always held be back from doing going this route initially was that at the end of the day, it's not actually insurance...so you're trusting the members to do their part. since we finally took the plunge, it has indeed saved quite a bit
as already mentioned by others, sometimes it takes a while to get reimbursed. we so far have been able to pay for our needs and wait the 2-3 months for the reimbursements to come. sometimes there's 1 or 2 who don't send their monthly share, and it simply delays things (not that big of a deal). if you had a really large expense that you couldn't immediately pay out of pocket, you'd have to just work with the hospital (or whatever care provider) to be patient on getting paid
I think in theory it's possible that needs/expenses could exceed the monthly inputs from members - i know ours has had that happen before as there have been "rate" increases (it's voted on by the membership). I'm not sure how they handled the needs though (I think initially they are prorated...maybe you get 9/10 of your need initially and the rest made up in future months). i'd have to go back and read up on how they handle that. that said, this is something I'd investigate with whatever sharing group you're looking at (and maybe I should go back and review ours)