III-nitride waveguides featuring AlInN claddings and GaN/AlGaN quantum wells (QWs) offer promising perspectives for applications in many fields of short-wavelength photonics. Thanks to their nearly lattice-matched nature, these structures exhibit an excellent material quality, leading, e.g., to strong light-matter interaction in such QWs, and several promising phenomena. In the low carrier density regime, the strong coupling between QW excitons and waveguide photons results in propagating hybrid light-matter particles, called (exciton-)polaritons, which combine photon-like propagation and exciton-like interactions. These interactions lead to a strong optical nonlinearity, which could be useful for integrated all-optical devices. Due to their strong exciton binding energy (~40 meV in the present structures), III-nitride devices have the potential to maintain these nonlinearities up to room temperature. In the high carrier density regime, a GaN/AlGaN QW electron-hole plasma can provide gain to an optical field in the UV, which can be useful for realizing near-UV laser diodes and semiconductor optical amplifiers. The performance of current UV devices featuring AlGaN claddings is limited by poor material quality. The improved structural quality of waveguides with lattice-matched AlInN claddings could therefore circumvent these issues. This study aims at an in-depth investigation of the optical properties of III-nitride waveguides with AlInN claddings and GaN/AlGaN QWs grown on freestanding GaN substrates. In a sample with an active region that was optimized for strong exciton-photon coupling, we observe propagating polaritons in the low-density regime. A sample with an active region that was optimized for homogeneous near-resonant excitation with a 355 nm laser shows elevated optical gain in the high-density regime. The nearly lattice-matched nature of the entire structure leads to a high structural and optical quality. We found inhomogeneous broadening values between 8 and 11 meV, and a standard deviation in the QW emission energy well below 1 meV over a 50 × 50 µm$^2$ area. We calculated the band structure and transition energies of the QWs using self-consistent Schrödinger-Poisson k ·p calculations and found an excellent agreement with experiments. The waveguide polaritons feature a normal mode splitting as large as 60 meV at low temperature, thanks to the large overlap between the optical mode and the active region, a polariton decay length up to 100 µm for photon-like polaritons and a lifetime of 1-2 ps. These decay lengths and lifetimes are limited by residual absorption occurring in the waveguide. The large normal mode splitting and elevated in-plane homogeneity are important assets for the realization of polaritonic integrated circuits. We also demonstrate optically-pumped waveguides exhibiting narrow bandwidth (3.8 nm) optical gain around 370 nm. Due to the high refractive index contrast between the cladding layers and the active region, the confinement factor is as high as 48% and net modal gain values in excess of 80 cm${¿1}$ are measured. The results agree well with self-consistent calculations accounting for built-in electric field effects and high carrier density related phenomena. As such, these results open interesting perspectives for the realization of more efficient near-UV laser diodes and semiconductor optical amplifiers.