"There are thousands of volunteers who donated their labour to Duo... Bit by bit all of our work was hidden from us as Duolingo became a publicly-traded company."

I started looking for alternatives when they added the weird character voices and I started noticing inaccurate pronunciation of kanji in my Japanese course. A lot of people on the message boards recommended Memrise, and it's been great! The official courses contain actual video and audio of native speakers, so I knew for a fact the pronunciation would be correct---even better than the old Duo voices!

There's also user-generated content, too, some of which might not be accurate, but most of the user courses I've found are pretty good. You can even make your own set and publish it.

(I haven't visited the site in a few months, so I can't guarantee it'll be exactly as I found it, but I doubt it has changed much)

And depending on what languages you're studying, you might be able to find some good ones dedicated to your language if you do some digging. For Spanish, I used SpanishDict, and for Japanese, I used Kanshudo (both are freemium, with more restrictions than Memrise)

My library offers Mango Languages.