There’s deeper history with how DF has usurped the fandom-run drama subbing community that I won’t really get into here, but just be aware that it’s there as an undercurrent to the frustrations with DF in general.
The subs themselves are… okay, I guess… but it’s more of a “here’s the gist” instead of a faithful interpretation. Like if you’re going to read the Bible and are happy with “The Message” translation, which makes for a good read, but isn’t as accurate as, say, the ESV translation (which maybe some don’t like because it’s not as easy/fun to read).
There’s often elements where the subbers decide how to translate something to interpret a scene but they actually are wrong by taking away from the actual wording. The Korean language is delightful in the way it plays with wording (so many puns!) and also there’s a vagueness to the language which often leads to misconceptions that further plots. I’ll direct you to my proverbial straw that broke my camel’s back (and let’s just say I never in my wildest dreams imagined I’d be arguing on the internet about condoms in a kdrama, but there you go).
Personally, I’ve always been fond of the fan-subs. My first encounter with dramas was through the WITHS2 community (and you’ll notice that on a lot of the pre-2011 dramas, DF uses fan-subs taken from communities such as those), and today my preference are the Viki subs. Of course, fan subs aren’t perfect, either, but I feel that they work hard to keep true to the original wording and intent of scenes than the so-called professional subbing of DF.
I always really want to support DF just because they’ve helped to make dramas so much more accessible, but their business practices, attempts at monopolies, and mistakes in subbing give me serious pause.