I very rarely use the app except to set battery mode or check charge cycles.
Our experiences differ on several fronts. I'm still tweaking my default (Universal) assist modes and TCU data layout, all of which must be done in the app.
I've also opted to forego a Garmin or Wahoo for now and use the app on a phone on my bars for ride recording, basic mapping, ride analysis, and ride transfer to RideWithGPS for further analysis.
Not ideal, of course, but it works well enough that I'm in no hurry to spend $400+ for a separate computer. Better uses for that money right now.
The MM display is too far down to be much safe use while riding, other than a quick glance of battery level. It's a good way to crash looking down at the MM display too much.
Agree that you shouldn't keep your eyes on the TCU too long, but the glances I feel safe with are plenty long enough to make the TCU a very useful display.
For starters, the TCU's bright and crisp and easy to read in any light. Vastly easier to read than my phone screen. This is my semi-custom page 1 of 3, the startup page. The 0.0 is ground speed.
Custom page 2 of 3, the one I glance at most by far. The TCU's capable of showing many if not most of the data items you'd want a Wahoo or Garmin to show you, though not all at once.
Current custom page 3 of 3. The info I choose to put here changes from time to time. The app makes it very easy. More pages are available, but I prefer just 3.
NB: My Vado SL 1 is a 5.0. Not sure what features you'll get with your 4.0.