The injection of spin polarized carriers in semiconductor lasers greatly modifies the device operation. Although the vast majority of spin lasers are based on semiconductors with zinc-blende structure, there is a recent exception using nitride-based compounds with wurtzite structure, which still lacks a reliable theoretical description. In order to address such deficiency, we investigated (In,Ga)N-based wurtzite quantum wells following typical device geometries. The small spin-orbit coupling in such nitride materials allows the simultaneous spin polarization of electrons and holes, providing an unexplored path to control spin lasers. For instance, based on microscopic gain calculations[3,4] we found a robust gain asymmetry, one of the key signatures of spin laser operation. In addition, we combine these microscopic gain calculations with phenomenological rate equations to investigate threshold reduction features. We show that the lasing threshold has a nonmonotonic dependence on electron spin polarization, even for a nonvanishing hole spin polarization. The complementary information of these theoretical frameworks provides a powerful predictive materials-specific tool to understand and guide the operation of semiconductor spin lasers.  Holub et al., PRL 98, 146603 (2007); Lindemann et al., APL 108, 042404 (2016); Rudolph et al., APL 82, 4516 (2003); Frougier et al., APL 103, 252402 (2013).  Cheng et al., Nat. Nanotech. 9, 845 (2014).  Faria Junior et al., arXiv:1701.07793 (2017).  Faria Junior et al., PRB 92, 075311 (2015).  Lee et al., APL 105, 042411 (2014).