Well as it's largely unchanged (actually canni change that to refined, I still like the design) from the previous 2 years, yes, it's definitely more slippery than any other iPhone they have ever made particularly in the Plus sizes.
It's the only real nit pick I've ever had with this design, they have concentrated on making it feel better in the hand and it does. But rounded edges on anything are only naturally going to glide across a surface easier. You get used to it of course, not enough grip and it will slide out of your hand, too much grip and it's like holding a wet bar of soap. Like anything I suppose it's just adapting to it.
I kept my 6+ in a case for this exact reason, which just makes it bulkier and defeats the purpose of having nice design in the first place. So this time round to hell with it, if it falls it falls, it'll add character 😀