chabadgal,
The issue with smartphones isnt that theyre necessarily bad, it is that they are new.
Technically, if you dont trust yourself with a smartphone, you shouldnt trust yourself with having chicken and cheese in your fridge together, shoudnt trust yourself with having electricity in your house during shabbos etc.
Smartphones are very different from chicken and cheese. Smartphones have only been around since 2007, but they have already caused massive cultural changes; affecting the way people interact with others, how they ingest and consume information, and how they raise children. There are undeniably some benefits to the technology: it’s a lot harder to get lost while traveling, there’s a lot of information to learn right at your fingertips, etc. But undeniably the devices have a tremendous psychological impact, even before we get to the yetzer hara issues DaasYochid mentioned. And we don’t even know the impact.
Cars have many benefits, but would you trust a driver who takes it for a spin but can barely see out the windshield?