One is that they put the Internet in everybody's hands, literally.
The other, though, is that by virtue of the interface, both display (tiny) and input (shitty, to put it mildly), the effective IQ of those participating, regardless of whatever it was initially, is severely penalised.
When I'm typing at a keyboard, I can look at the words I'm typing, or the source I'm typing from, or just off into space as I'm thinking my thoughts ... and be reasonably assured that what I think I'm typing is what's actually appearing on screen. And if not, editing to correct and fix issues is reasonably straightforward. If necessary I'll switch to a Real Editor (that is, vim, Vale Bram Moolenaar) which is another quantum leap beyond in-browser textbox editing.
When I'm typing at a virtual touchscreen keyboard, I am staring at the keys themselves and trying to hit the the keys I'm actually intending to hit. I'm not monitoring the output (which invariably has errors), I'm not looking at source text, I'm not thinking and composing my thoughts.
And then editing what I've written is also painful.
The resulting typos, losses of thought, and general incoherence in my own writing absolutely pains me to look at. From what I can tell, other people seem to suffer similarly.
I've given up using mobile devices for input (unless I can use a hardware keyboard, and even that rarely), and ... frankly it's a much improved situation.
Smartphones shallow the mind in many ways.