Slack should count as work too while commuting. I'd prefer emails but now you are de facto on call when you have Slack on your phone.
Problem is that not everything on Slack is... "productive"... (Though the same could be true for e-mail.)
I feel like companies use Slack exactly for this reason...it tethers your employees to a very intimate possession-your smartphone and it's far more invasive than a phone call or email and your are expected to respond immediately.
It has all the dynamics of any email or regular office conversation. Somebody has a grudge in the office and want to make you look bad for remote working? Call them and immediately hang up and tell managers 'so and so isn't responding in real time' or take a convo out of context where emoticons play a far more conflicting messages to the receiver. For example, somebody using a animated flashing parrot and mistaking it as a slight.
I feel like Slack has been more times counter-productive than advertised.