I think it depends on you AC and spec. My gunnery problems only had trouble with one quest. My knetic combat breezed through. My scrapper scoundrel, however is having a harder time. I actually needed a buddy to help out with 3 or 4 quests. And i never had any problems soloing any planet until Voss. I definitely notice they hit harder and have more HP. It is doable, but I do notice the increase in mob difficulty.
I am not sure about everyone else, but my concern is mainly because i soloed correllia with all 3 so easily but then voss just provided more challenge. Mind you I am that guy who goes to most of the planets 1 level below the required level .