The configuration of the USB-connectors turns out to differ quite a bit between the diverse models. A few cheaper boards do not offer USB 3.1 - for which you still need external controller chips with Z370 - and the ASUS ROG Strix Z370-I Gaming only offers USB 3.1 via an internal header. This unfortunately means that if you do not have a case with a connector suited for this, this header is of no use to you.
Something that we have not come across before, but are now seeing with both ASRock and Gigabyte, is that the header meant for USB 3.1 is connected to a USB 3.0-controller. With that the speed of the front connectors is reduced to 5 Gbit/s and practically speaking the added value of the new header disappears. However we cannot judge this too harshly, because there are only a few cases with this connection and the question remains how important it is to have a front 3.1-connector. It is potentially confusing. We can imagine that the newer header, which is a lot more compact than the old USB 3.0 header, will eventually become the standard connector for external connectors and will be able to transfer multiple speeds as with Type-C.
Aside from the internal headers for USB 3.1 (whether or not gen2) every board also has one or more old-fashioned USB 2.0 and 3.0 headers for connecting the front panel of your case.
|USB 2.0||USB 3.0||USB 3.1||USB Type-C front|
|ASRock Z370 Killer SLI||0||6||0||1x 3.0|
|ASRock Z370 Extreme4||0||4||2||1x 3.0|
|ASRock Z370 Gaming K6||0||4||2||1x 3.0|
|ASRock Z370 Taichi||0||4||2||1x 3.1|
|ASUS Prime Z370-A||2||2||2||0|
|ASUS ROG Maximus X Hero||2||4||2||1x 3.1|
|ASUS ROG Strix Z370-E Gaming||0||4||2||1x 3.1|
|ASUS ROG Strix Z370-F Gaming||0||4||2||0|
|ASUS ROG Strix Z370-I Gaming||4||4||0||1x 3.1|
|ASUS TUF Z370-Pro Gaming||2||4||2||0|
|Gigabyte Z370 Aorus Gaming 7||0||5||2||1x 3.1|
|Gigabyte Z370 Aorus Ultra Gaming||2||4||2||1x 3.0|
|Gigabyte Z370-HD3P||2||4||2||1x 3.0|
|MSI Z370 Gaming Pro Carbon AC||2||4||2||0|
|MSI Z370 Godlike Gaming||0||6||2||1x 3.1|
|MSI Z370 SLI Plus||2||4||2||0|