Notification channels are great when an app actually uses them (properly). Of late I've been seeing many apps sending spam thru the main channel, or not describing/using channels appropriately, all of which makes them kinda useless.
My folks live overseas and they often miss my chat messages because it gets lost in a sea of junk notifications. When I visited them last time, I tried to use notification channels to filter some of it out, but it was useless - there was basically no way to just filter out the junk notifications (without disabling notifications completely) for most of the apps they were using.
What we really need is a generic notification filter, where I can define my own text/regex filters, or maybe even employ some bayesian-style spam filter. Heck, I'd say Google should add a "mark as spam" option to one of the notification options, and if an app gets enough of these, the devs should get a warning or delisted from the Play Store.