I would never consider it "secure enough" to be using for anything truly sensitive.
Even if you
turn off all tracking and telemetry features, Windows 10 still tries to phone home a staggering
5,500 times in 8 hours. The additional use of third party tools designed to stop this did help a bit, but these researchers still reported 2758 connections in 30 hours.
Imagine this situation from the perspective of software companies making products for Windows, using Windows 10 development machines (or non-software companies competing with Microsoft or with someone MS sells your data to). Microsoft just pushed up their running costs a great deal, as those competing companies must now protect their data from Microsoft and the other companies Microsoft sells user data to. Would it be cheaper to ditch Windows completely? Maybe this wouldn't always be viable, but the incentive to support Windows as a platform is reduced now that Microsoft are behaving like this.