In season 17 does LoN buff work against season journey set rewards?

LoN builds are generally not that strong, but it does free you up to use 2 ancient rings to increase the benefit of the passive seasonal buff.

In your specific case, the build guides you are following are just throwing the seasonal buff out the window because the set bonuses are so crazy strong that there's no point in building around ancient items only.

To answer your question in the title of your post, yes, it does work against each other. Seasonal buffs were never thought to coincide with seasonal rewards, these two things were built separate from each other.

As opposed to FoxMcCloud, I'd say that LoN builds are way stronger than most set builds. In fact, LoN builds are the strongest push builds for this season according to Rhykker who is right most of the times.

Not all set bonuses are "crazy strong". Some actually are that bad, that you would rather wear a few ancients than using the sets. This season, if you want to push really high in greater rifts, you mostly want to get your hands on the season journey set reward to farm ancient items effectively.

It is true that they season buff and season journey reward work against each other, but you can also use that as advantage for an easier farming for any LoN build.

The season buff is not useless if you aim for really high greater rift pushing.