Ecobee disconnecting from Smartthings

Yes, a good design for the DTHs makes a lot of differences. If your DTH uses a lot of state variables, its reliability will greatly suffer over few days or few weeks.

You may check the ecobee status page if there is a major ecobee outage going on.
There have been more ecobee issues recently as ecobee has been busy with upgrading its APIs since the introduction of the ecobee Switch+ (for example, June 18 and few days before).

You may want to subscribe to the ecobee status reporting at their page to get notifications.

As you can see, there has been no major ecobee issues in the past week…

In the last year, with My ecobee device (and despite the recent ecobee issues), I have not re-entered any login information for my ST account or any of the ST accounts that I personally monitor as part of my support packages.