I'm not sure where you are flying from, but I think it's always worth it to find out the cost of doing what they call an open jaw ticket -- flying from home to Lisbon and then from Santiago to home. In your case it would probably involve a round trip to Madrid, with an extra flight to Lisbon in the beginning and from Santiago at the end. You usually can't find these ticket online and have to get help from a real person on the phone, I believe. I have found on several occasions that the additional cost of those extra segments is trivial. For instance, last year I flew from US to Valencia and Santiago to US for $100 more than it would have cost me to fly round trip from US to Madrid.
There is a bus from Santiago to Lisbon, it is a LONG boring ride, with a 45 minute stop in an uninspiring highway cafeteria. The bus leaves around 11 in the if my memory is right and gets to Lisbon around 6 or 7 hours later.
I think that many of the different experiences on the roadside stretches have to do with time of day and day of week. I, like Stephen, always felt like I had a wide paved shoulder on the busy roads and never felt like my life was in danger. But as they say on TV, your results may be different. Buen camino, Laurie