A google search will reveal a lot of negative opinions about Anki. To summarize some of them:
A very common complaint about Anki is that it assumes, and effectively requires, daily use of the software. As described in Language Hackers And “Polyglots” Are Full Of Shit:
A ... severe problem is that you must use the program every day, or else your reviews build up to such a level that it may take you hours to catch up. If you go on a one-week vacation, you will come back to a mountain of cards that will take you half a day to complete.
I also complained about this problem in my own blog post about Anki shortcomings, Where Anki falls short:
If, due to circumstances (such as vacation, or laziness), I fail to study for a few days, I end up with a huge back log of review cards.
Another common complaint is Anki's rigidity. Languages aren't clean and tidy, but Anki assumes, and effectively requires, a "clean and tidy" data set. As described in I Quit: 2+ Years of Anki and the (Near) Impossibility of Learning Languages:
Take the word 소수 (so-su) as an example. It has many meanings, among them: a minority, a decimal, and a prime number. These different meanings cannot be distinguished by looking at the word. Only context can tell us what the meaning is.
The author of that article describes several alternatives he tried, but found them all wanting.
Anki has no way of tying related cards. It does provide the option of burying cards generated from the same note when you study one, but doesn't provide any way of tying related notes together. Again from my blog post:
[Anki] will bury the reverse card which asks me to translate “cat” to “gato,” [but] I may have another card translating “gatito” to “kitten”, and this should be buried as well.
A further complaint of mine, with regard to burying cards:
Once cards are sufficiently mature (with a delay of months), burying a related card for just a day is not sufficient. If I get a review card about some especially obscure word after a 6 month delay, I don’t want to see the reverse card the next day. These reviews ought to be staggered. Perhaps by as much as 3 months (or half the review time).
Finally, Anki's UI sucks. I think I can say this is an objective truth, but perhaps somebody likes it. I almost gave up on Anki before I even started using it, because I couldn't figure out how to create cards and card templates. After a couple months, I gave it another go, and was successful. But I would be amazed if Anki's non-intuitive UI doesn't drive away a huge number of potential users.
Some people, such as the authors of Language Hackers And “Polyglots” Are Full Of Shit and I Quit: 2+ Years of Anki and the (Near) Impossibility of Learning Languages, seem to be of the opinion that these problems are fundamental to Spaced Repetition Software as a tool for language learning.
Personally, I take a different approach, that SRS can still be a valuable tool for language learning, but that these drawbacks are specific to the implementation (of Anki, as well as SuperMemo upon which Anki is essentially based). As such, I'm currently working on my own SRS software to address some of these deficits.