While I can't vouch for voiczy, Duolingo is in the business of user retention and engagement in order to meet investor demands...language learning is the hook.
Duolingo is notably POOR at real language acquisition.
Are there other apps that are more effective?
Yeah I get that. I don't think my son is ready for something effortful like LingQ. So, even if Duolingo is only a tenth as effective per unit of time spent, it's better than nothing.
Will you please provide some additional context here as to why and how Duolingo is poor at language acquisition? What software is better at language acquisition?