IIRC, there are a few variants of the Tourney series. Anyhow my old hardtail has Tourney tx50 or something. It shifts fine. Like anything, adjustment of the cable is critical. RD rarely acts up. It's the FD even though its task is to push the chain back and forth that has most issues. And it is the part where mfg skims on because often times consumers hone their eyes on the Rear Derailleur on the spec sheet.
BTW, isn't plastic supposed to be lighter than metal?
Is the Tourney RD larger than Altus, Deore and XT RDs? Never noticed that.