Just my own experience and take: Duolingo years ago used to be fairly okay. There was a lot of repetition, a lot of freedom to space the reptition as one saw fit, and move on to new lessons in between. There wasn't much grammar from Duolingo itself, but most exercises/sentences had a small discussion forum in which more experienced users could teach and explain the why behind the practice. This was the case with all languages, I remember the Ukrainian course was lovingly tended to almost entirely by a couple of volunteers, people who cared about the work almost as Michael and Saki do here. (I mostly studied Russian and Ukrainian when I was there but did do some Japanese before leaving.) As for the Japanese course, it wouldn't get you very far, but it was a good place to start, learn the basics, kanji was integrated in naturally, and could get a lot of reading/writing practice.
Then Duolingo started updating.
The tree became the path. Linear. Less freedom to practice what you want and move on where you want.
The normal ordinary voices were removed in favor of a handful of characters around whom everything in Duolingo became centered. The characters were definitely a ymmv situation but if you weren't into it, tough luck they were everywhere.
Worst of all the sentence discussions I mentioned above, gone. All forums even, gone. First locked, then even worse taken down entirely. A treasure trove of crowdsourced information erased. No real effort to teach grammar or any way to get an explanation from a human as to why the language does what it does. Still Duolingo c. 3 years ago still could work as a series of reading/writing practice exercises used in addition to other resources.
But Duolingo kept updating.
The hearts system once on mobile only, became introduced on to web Duolingo. Much of what was on web Duolingo was replaced with the weaker mobile version. (There was a way around it by registering as a school. That loophole may have been closed, don't know, I bailed by then.)
Ads. Everywhere. Get Super, buy Super, buy an owl plushie and buy more Super.
Writing practice got nerfed by Duolingo forcing everyone to use multiple choice bubbles. Pretty much any sort of enjoyable customization by then had been removed to push everyone into a very similar linear experience. Any feedback opposing any change is famously ignored. Duolingo takes pride on not listening to the people only their "experts" and "metrics."
To end on a positive that last point is what makes Renshuu great by contrast. We're seen as people, not as numbers and metrics. Not everyone learns the same. Not everyone has the same individual goals even if they share the same overall goal of learning Japanese. Here if you want more practice in a specific category you can add that schedule or customize the settings to focus on the type of question you need. There's freedom here, more than Duolingo ever had even back when it was better. There's a serious effort to teach grammar. There's opportunity to structure study toward JLPT standards if the user wants. There's a garden where you can buy and place cats.
There isn't an ever-present demand to buy Pro here, and yet look at that I bought pro as have many others. Turns out just having a good service with a promise of more (and also the availability of a lifetime option) encourages people to want to upgrade more than in your face advertising.
but tldr, Duolingo updated itself into a weaker choice by my experience. But ultimately I still stand by my belief that not everyone learns the same. It could work for others, that's not for me to say. Maybe there's helpful stuff that's been added since I ditched the place. Personally in addition to Renshuu I prefer Cure Dolly on Youtube, TanoshiiJapanese and a simple little kanji site called Realkanji.