Are the effects of Lucio's secondary fire diminished or increased for certain heroes?

Solution 1:

No.

Related

In my testing I hit every character with concussive blast on Pharah, and each traveled approximately the same distance. I also tested knocking back Roadhog, Tracer, Zenyatta and Reinhardt with Lucio's knockback, with the same results. The knockback from Blast of Sound is agnostic of character 'weight', or distance from Lucio. If you're in the area of effect, you get knocked back X units.

One small caveat to this is Lucio himself- he traveled further than any other character. However, this is not weight related. When knocked back, Lucio traveled the same distance before making contact with the ground, but his skating shoes carried him further back, as they preserved the momentum from the blast.

Testing information:

I conducted this test in King's Row, with the Defender being the class to knockback (Pharah and Lucio), and the Attacker being the class to get knocked back. To the left of the Attacking spawn (from Attacker's PoV), there is a manhole cover near a phone booth. The defender stood on the manhole, with a spray placed the same distance as the phone booth for the Attacker to stand on. The knockback direction went horizontally across the Attacking spawn. This ground provided a long, flat surface with no ceiling in order to keep the knockback as unaffected as possible.

Solution 2:

While the accepted answer is correct, it's somehow "incomplete". Yes classes do not passively react differently to Lucio's knockback, BUT some class ability CAN alter the knockback distance.

https://www.reddit.com/r/Overwatch/comments/4o4cw2/lucios_knockback_is_affected_by_certain_crowd/

A good example is D.Va, it get pushed far less while she's shooting.