Mos Eisley is a sorta scummy place, I guess there are some rough characters there who would pull a blaster on you at the drop of a hat (not noble hero Han Solo though. No, he would only pull a blaster to defend himself). However, Obi-Wan telling Luke he would never find a more wretched hive of scum and villainy was more than a little disingenuous, considering there's a FAR more wretched hive of scum and villainy located ON THE SAME PLANET.
That's right Jabba's palace, a hive that's 20 times more wretched and scummy and villainous. There's a fat slug guy who keeps a human being frozen in carbonite as a conversation piece, regularly abuses his slaves, and has no problem feeding his servants to the giant cave monster who lives in his basement. Also, his idea of a good time is throwing people into a toothy sphincter in the middle of the desert.
Compared to that, the Mos Eisley Spaceport is practically a neighborhood Applebee's.