I have heard from friends that the TS3+ is slightly less reliable than the TS3. Initially -- 2017-ish -- we had challenges with the monitors (Dell's) being weird. We updated the firmware on the monitor (no fix), and updated the cables to newer DP cables (Fixed it).
Dell did not make a thunderbolt monitor in 2017. I'm still not sure if they do. You might be referring to running DisplayPort over USB 3 (with a type-c connector)
USB 3 video is a troublesome way to connect a monitor, with a lot of vendor incompatible techniques for signaling sleep/wake, charging, and obscure limitations on resolution/refresh. For example, for years Dell stated that their USB C monitors were not compatible with Apple devices.
Thunderbolt 3 seems far more reliable, in my experience.
Wasn’t implying they were TB; the dock has DP + USB-C (w/ DP alt mode) — those were the monitors connected to the dock (one via DP-DP, one via USB—C-DP cable. The one that ‘fixed’ it was the DP-DP cable)
YMMV, of course.