I'm not sure how it compares, but I recently went 4k@60Hz with a Dell XPS 15 + docking station.(Dell WD15) + Vizio D40u-D1 (40"). That uses Displayport out of the dock, and a Plugable active DP to HDMI adapter before going into the TV as HDMI as 4k@60Hz. I find it surprising that the monitors, which likely cost 2-3 times what that TV cost ($400) don't support it over HDMI (but then again, the TV doesn't even have DP).

That said, Dell did have a USB-C dock, the TB15, which was plagued by a lot of the same things I hear about USB-C from the Macbooks with USB-C. Wifi and other components stop working intermittently, and on the dock, it flickered intermittently when connected at 4k[1]. Dell just released a new dock, the TB16, and there's a BIOS update for my XPS 15 that says it fixes some docking issues.

1: I seem to recall someone mentioning this was related to the firmware in some connected peripheral, but I'm not sure.

That's because you don't need an adapter to use DisplayPort with TB3 (on the MBP). Just purchase the correct [1] cable; the MBP implements DP as an alternate mode.

[1] https://www.amazon.com/gp/product/B01EZH7CKO/

That cable isn't available on this side of the Atlantic, local Apple store only had a faulty DP adapter. For some reason not even the german Amazon had the proper cable in stock or listed.

I want ONE CABLE for everything. That was the promise, I thought. I want 2x4k@60hz + power + 3 USB3 with one connector going to the tbMBP. Apparently this is not possible.

Instead you have 4 identical ports that all do the same things poorly.

The reason 4k monitors only support displayport is because current HDMI can only do 4k at 30hz. DP is a better cable with more bandwidth. Expect DP to overtake HDMI in the future.

I have a Vizio P55-c1 on my desk and with a GTX1080 and a BlueJeans HDMI cable, it works fine at 1080p@120hz or 4K@60hz. It's HDMI 2.0, so another case where just because it plugs in doesn't mean it'll work: https://en.m.wikipedia.org/wiki/HDMI#Version_2.0