Personally, I find the interface is extremely functional; the ability to have deck hierarchies to be a massive feature, not a bug; the WYSIWYG being the default being obvious given the intended audience, but one can still easily edit a textfile and import it or edit in html mode directly if desired; converting something into latex math is as simple as enclosing it in "[$] ... [/$]" and hardly the nightmare it's portrayed as; and finally potentially hacky plugins is a feature, not a bug: occasionally you have a very specific problem and some kind soul creates a solution for you, which may be functional but not the most aesthetically pleasing. That's fine. Anki is a bazaar, not a cathedral, and plugins have ratings and reviews which you can consult if necessary.
I have tried many different flashcard solutions, including hacky text-based ones, and I always return to Anki. Despite the fact that most other tools in my stack that I swear by are terminal-based.
If you're potentially interested in his project, you should evaluate your interest based on how much you think like him. If his complaints aren't yours, no skin off your back. Just ignore him. If they are, read farther.
However the reason I find it off-putting is because, as someone who generally lives in the terminal, and Anki is one of the few remaining GUI apps I rely on, I actually "would" have preferred a decent terminal alternative with similar features. But introducing the alternative by saying how much Anki sucks immediately puts me off when all that criticism doesn't resonate with me.
It literally works as anti-promotion here: if Hashcards promotes itself as missing all those features of Anki which I think are great, and my time is limited, then I have much less of an incentive to invest the time to check it out. Which is ironic, because in reality it may be great (like most of his other work) and actually suit my use-case really well.