There are two MASSIVE issues with voicechat.
#1. Voice chat is far harder to moderate and detect toxicity in than plain text chat. This should be fairly obvious. The only possible ways to moderate voice chat are either human labor (would be literally impossible to do in any sort of timely fashion which was also one of the biggest reasons the Tribunal was a horrible failure) or voice-to-text and then recognizing from there, except that has huge problems with recognition currently.
#2. This creates a huge gap between players who cannot/do not want to/will not use voice chat and those who do. This should also be fairly obvious. Communication is incredibly important and voice is far more useful than text chat for multiple reasons, speed of communication being one of the biggest and the other being not needing to stop to type. Players who use it will have a huge advantage over those who do not. There will be many players who will not use voice chat either because A, they cannot (no mic/etc) for one reason, B, cannot for another reason (sick or people over or whatever), or C, (has some sort of problem regarding talking such as shyness or a complex or whatever) another reason, among other reasons. This creates an unreasonable gap. At least with people using stuff like Discord, it meant it was up to the players to decide.
#TL;DR: Too hard to punish people with what they deserve for voice-chat toxicity and creates a gap between those who use and those who don't in performance.
Too much to handle Riot?
Crowd source the Voice chat Moderation, offer IP(Rewards) for the work?