First, it is called a VISUAL novel, so voices are ‘just’ a bonus to the overall experience.
Voices are good IF there are talented actors, otherwise it will ‘pull down’ in the experience and people will turn them off.
It can be a challenge to pick the right voice, as it needs to match the character and be clear in its expression… just like written text.
If there is much speech, it will be expensive to add voices which includes workload. If there is only a little speech, will it add much to the VN?
In my VNs I do not use voices because I have many characters and much dialogue.
All the best!