[1] https://community.ubnt.com/t5/EdgeMAX-Stories/EdgeRouter-Inf...
My NEC router had no default password and required you to make one up yourself on initial setup.
Your grandma is getting a Comcast router and probably doesn't know it can be logged into and configured or how to do it.
UBNT's new-ish home router series (AmpliFi) doesn't have default username/password- it needs to be set-up before using. I do think it's possible to have an open-network default configuration, but the LCD will nag you to set-up the device, and the first step of the setup is choosing a password (both for management and WiFi).
Disclaimer: I work at UBNT.
Seems fairly secure to me.
On the other hand, a Midcontinent Communications (aka Midco) tech told me the password I wanted to use wasn't secure enough and brainstormed with me for a couple minutes on good SSIDs and passwords while he showed me the web admin interface on my laptop. I was very pleased with his visit and called the local office afterwards to pass along kudos!
Nope. Anyone within range of your wifi router can connect to it and most possibly the first username and password that they will try is admin\admin or admin\password
I called the support line and got through quickly to an admin who could change the password and SSID for me (unless you pay for a public IP, the mgmt interface is locked down). I mentioned that having such a short, all numerical password would mean that any access point they set up would be trivial to crack. Just wardrive looking for similarly named access points, and you'd be able to jump on their connection in just a few minutes. He didn't seem to care, which is too bad.
Plugged in my 'scope and started probing some debug headers that looked a lot like they'd be for UART, check if one is Tx and is sending out data, figure out the baud rate, hook up Rx, Tx, and GND on my UART dongle to the correct headers, and modify the bootsting in Cisco preboot to spawn a serial console which landed me into busybox as root :)
Followed by the use of >=$100 of hardware and some not-beginner skills. Snark aside, I highly recommend anyone remotely interested in what is going on in your modem/router to have a go at this. You don't need the scope if you're okay with trial and error and it's pretty hard to break anything as long as you don't connect the 3.3/5V line to start with.