First off, just to add information to the mix, Dual-Wielder Feat and Two-Weapon fighting allowing 4 attacks with Nick, all using mod, is one of the intended results of the feat, per Crawford by way of Treantmonk's friend
Update on today's video: Someone I know asked Jeremy Crawford about Dual Wielder at Gencon, and here's how it works: It provides a single bonus action attack...
www.youtube.com
Looking at numbers, with no current access to a full book.
At level 5 -
Maul and GWM --> 2(2d6+4+3) or 4d6+14 or 28 damage with a single feat. If you drop or crit, you can bonus action for another 2d6+4 getting a "spike" of 39 damage.
Shortswords and DW --> 4(1d6+4) or 4d6+16 or 30 damage with a single feat. No real way to get more, and your bonus action isn't free.
Polearm+PAM+GWM --> 2(1d10+7)+1d4+4 gives us 24.5 per strict RAW, with a reliable reaction attack for 1d10+4 and a spike boost of 1d10+4. So a fairly reliable 34 with a high end of 43.5
Shortsword + Rapier + DW --> 2d8+1d6+12 or 24.5
So, I think it is fair to say that DW + Nick giving 4 attacks is pretty strong, with the two major styles of GWM keeping up, with the Rapier/Shortsword is falling behind without accounting for accuracy (it is possible that Vex makes a difference). Not bad overall.