Every town is. Any flying bosses can cross over towns.. Most popular one is Jewelscalen since it does AoE damage and AoE defense debuff.. Hence, alot of people get killed when they are not even aggroing the boss..
Water bosses are obviously can't survive in land.. Land bosses can't cross water but they can go all the way through lands just to get killed by Guards NPC in every major cities (minus Dreamweaver and Thousand Streams).. Some small towns have guards also in it..
Example of a boss gets into West Archo:
https://2img.net/r/ihimizer/img709/6286/wdwb.jpgJewelscalen having fun in Raging Tides City:
https://www.youtube.com/watch?v=3WUtZjJD3kwSafe Zone just indicates that you can't get PKed by other characters.. Safe Zone doesn't mean that you will not die cause bosses can get into towns anytime.. Also, afking at 78 height wouldn't help since flying boss like Jewelscalen can easily be lured to 78 height..
Sooo, no safe place in PWI.. Safest is just to log out your character =)