Defense can be hard, but that’s more due to the nature of the game mechanics than Juri’s abilities specifically. There are a lot of ways to attack in this game, and there’s no throw invincibility on wakeup or out of blockstun or hitstun, and the throw tech window is short. This means you have to constantly be wary of cross-ups, overheads, lows, throws, armor-breakers, frame traps, etc. Every character has strengths and weaknesses in countering these various options. For example, Ryu’s main wakeup option is his Shoryuken, which may seem really good due to it being nearly impossible to do safe meaty normals or throws against it, but it can be still be crossed up, stuffed, blocked, or made to whiff. This makes it either a big risk or completely useless in certain situations. It’s really only a good reversal when Ryu has meter and is in range to FADC it in case it is blocked. One reason that Juri’s wakeup seems like it is not good is because it is actually balanced completely opposite to Ryu’s; her options carry the most risk against an opponent right in front of her. However, as long as she has meter, she can easily escape things like jump-ins and fireballs. The reason I said her wakeup is solid is because she almost never has to block on wakeup, whereas a character like Ryu often does. Backdash has 8 frames of invincibility, EX Senpusha has 6 frames of invincibility, EX Kasatushi has instant startup (and also cannot be option-selected against), forward dash has upper-body invincibility, cr.MK has upper-body invincibility, and cl.MK is airborne. With a good read, Juri can counter almost any offense without blocking.
I’ll address the Yun matchup in particular, since that seems to be what most of the complaints are about. Yun is a character with very strong rushdown abilities, but it’s actually not that bad for Juri because he has to work to get in on her in the first place. Juri has an advantage over him in the neutral game because of a few convenient traits, the main thing being her Fuhajin. She can whiff Fuhajin store to build up plenty of meter, and while it’s technically slightly slower than Yun’s meter-building with Kobokushi (qcb+P), it ends up working better in practice. Fuhajin has a faster animation and is more difficult to punish, so Juri can generally whiff them without worrying too much. Kobokushi, on the other hand, is always punishable because of Juri’s ability to dive across the screen with Shikusen. Juri can also hinder Yun further by releasing an LK Fuhajin, which goes right under Kobokushi. The more meter Juri builds, the harder it gets for Yun to get in on her, since she gains access to things like EX Senpusha and EX Kasatushi, in addition to some offensive threats like LK Fuhajin FADC and Fuharenjin. This forces him to attempt to get in on her early, but it’s not easy for him. Dive kicks are risky against Juri’s arsenal of anti-airs (st.LP, cr.MP, f.MK, cr.HP), and EX Zesshou Hohou (qcf+PP) is riskier against her than most because she is one of the few characters who can neutral jump and hit behind them on the way down (nj.HK). Yun ends up having to approach the matchup similarly to Zangief, by using footsies and pokes to work his way in. Unlike Zangief, however, he doesn’t have a large amount of health to spare while trying to get in. If he does get in, though, he gains a huge advantage in the matchup, and unless he makes a mistake, Juri has to make a random guess/good read to get out. Fortunately, the reverse is also true; if Juri gets in on Yun and corners him, she gains a huge advantage in the matchup, and Yun has to make a random guess/good read to get out. The matchup can often look one-sided because of this, but I don’t believe it’s actually in Yun’s favor overall.