A couple abbreviations for the glossary: GNSS = Global Navigation Satellite System; RNSS = Regional Navigation Satellite System. They're just what you'd expect: global coverage or not.
NavIC is the Indian RNSS; if you're not in its footprint it's useless to you. QZSS is the Japanese RNSS; if you're not in its footprint it's useless to you. WAAS is an RNSS designed to supplement GPS over the CONUS; if you're not in its footprint it's useless to you. GLONASS is indeed the Russian GNSS.
I saw a brief comparison of GNSSes that said GPS has better accuracy than GLONASS, and that Beidou and Galileo have similar accuracy which is better than both GPS or GLONASS. I'm not really equipped to properly test/validate/refute that claim, or even appreciate it. OK, it's cool that my cell phone can simultaneously use GPS (L1/L5), GLONASS (L1), Beidou (B1/B2a), and Galileo (E1/E5a). I've got a home time server which I haven't bothered to measure cable propagation delay or interrupt latency on because 1-millisecond time sync is quite sufficient for my uses (log file timestamps, correctly setting the time on my telescope controller, and programming the timer on my coffee maker). I would hope that my multi-constellation receivers are all smart enough to figure out which combination of satellites provides the best navigation solution..
Gary share his thoughts about why an absence of GLONASS support might be a feature; from my perspective it doesn't seem to offer any compelling advantages over GPS. It doesn't look like civilian multi-frequency are readily commercially available which would allow you to directly probe ionospheric effects. If you've got limited resources (tuners, correlators, DSPs, memory, CPU) in your receiver, maybe you want to use more of them for more useful systems. At least GPS has some usable L5 satellites. I dunno, maybe you have really restricted sky view in one location and GPS+GLONASS might give you just enough satellites to get a usable fix?