Ryanair are great as long as you don't expect more than they say they will provide.
And it's mainly common sense. Ryanair will fly you from Airport A to Airport B. Don't expect that to be city centre to city centre.
If you don't turn up in time to check in or board, you won't fly. These times are published and you agree to them. If they miss their take off slot, they have to pay for another one, and that means we all have to pay more
If your bag is bigger or heavier than they say you are allowed, then they will make you pay. That's hardly a secret. The sizes and weights are published.
Ryanair's prices are low because they can save money by running their business this way. If I get a lower price because of that, I'm all for it.
So, summary.
Book with Ryanair. The website will try to get you to buy all sorts of paid extras like priority boarding, car hire, baggage allowance, luggage, insurance. Just tick no. You don't have to buy these.
Check how far away the airport actually is from where you want to be. Stockholm Skavsta is a long way from Stockholm centre.
Make sure that everything you put in is correct, including dates/times/names/passport because if it is wrong, they can (and often will) make you pay extra to change it. Although I've never had them scrutinise my passport to see the numbers and expiry dates match, I have known of cases where misspelt names have not been accepted.
Check in online and print your boarding pass on a good quality printer. Several copies if necessary. If one won't scan, you'll have to go to the desk and pay to have a boarding pass issued.
Check the size and weight of your bag is within guidelines before you travel.
Turn up on time, with extra to take into consideration any delays. Advance flights may be cheap, but ones booked on the day are in the order of hundreds of pounds and they won't rebook you on a later flight.
Don't expect a refund if you don't fly, unless you have proof of a good reason, which usually includes providing some sort of death certificate.