I don't use echo auto but I use the echo frames for my hands free when in the car. It is more private since others cannot hear what is being said on the other end. Besides they are more comfortable because nothing is jammed down the ear canal like the echo buds.
I have Echo Auto. I bought one whenever they first rolled out. Used on both my car and bike. Love it. Now that I upgraded my head unit on my motorcycle, I have it built in to where the Echo Auto hub is now stationary in the car. I use an array of voice command services and each one to me has its own perks.
It's cheap enough to buy, very easy to setup and use. No cons that I can think of. The head unit that I bought for my bike is the Kenwood bt778hd if you want to have one of each to include Alexa already built in.
I never close my app on my phone. Although I have several hubs in my home. So whenever I'm traveling she's still active on my phone plus I'm using the Alexa Auto whether the unit itself or my stereo head unit. She will respond to whichever unit is closer. They do not interfere with each other and work as a team.
But yes, always open on my phone 24/7/365...