Just started playing regular games of 5th and an issue has popped up, my friend plays Templar and he takes the Emperor's Champion, one of the vows he can choose is Uphold the Honor, the wording in the Codex says that BT can't be pinned and can choose to have +6 inv. save but no cover saves.

My question is this radius based (like the IG's commaders inspiring troops) and is there any negatives applied? Ie Does he have to nominate before shooting which he is going to use or does he have to pick any units and nominate. Much appreciated for a quick answer (and hopefully avoid a heated argument)

Nope its not a radius, its everyone.
"no unit may..." as in all units follow this.
The big negative is no cover ever... Standing behind a tree, no cover, got interveing units no cover, gone to ground (not sure but probably not).It gets rid of the main safety net of 5th ed of 4+s everywhere all the time.

Only question I have is do dreadnoughts (in this new world of dreads getting invulnerables from anything that rolls damage) get affected by this (they are specifically said to be effected in the "accept any challenge"vow) as they all presumably lose getting hull down?

Well what's the rest of the rule say. Coz if it that's all it says then there is no radius effect.

For a radius it would need a range..
See also Vows on the black templar special rules page..(just about black templars not losing their powers when the champion dies..so if he can die and its still active..what do you measure from? )

You really never see this one around compared to the others.. its not the bestest.