It doesn't have to be doom & gloom - medical advances are amazing and neonatal / special care staff are some of the most dedicated professionals you'll ever meet.
It does depend on what stage they are born at, but also the reason why they were born early. If its just a question of them coming early (with no underlying medical issue), then thats obviously different than if the mother had issues with the pregnancy.
I went into labour (spontaneously) at 27 weeks and 4 days. The twins came 2 days later (so 27+6), weighing 2lb 6oz each.
There are lots of "what ifs" when they are on the special care baby unit - everyone talks about an emotional rollercoaster, and it is - but lots of premature babies are just there to learn to feed, regulate their own body temperatures (which prem babies find hard) and to grow.
Neither of my twins had any complications, even though they were so small, no major infections, no brain bleeds etc. DS was ventilated for 24 hours but were moved from intensive care to high dependency within 48 hours. They were 8 weeks old when they came home from hospital.
Some people talk about susceptability to infections - my twins weren't - even when they started nursery. They didn't have antibiotics until they were 2.
They are now 8 - they were completely discharged from the paediatric team when they were 2 (having caught up with their peers). They have no lasting effects whatsoever from their early arrival. I recently did some fundraising for BLISS and explained to people it was because they were early - their teachers / headmistress / parents of friends were all gobsmacked they were so early.