It seems, in WordPress, that it is not enough that a theme shouts about its support for RTL. And even when a theme gets positive reviews from a site like onlyrtl.com, that may not be the full story. The proof is in the pudding. And, because of such positive reviews, it isn't so easy to check before purchase.
That's my experience after purchasing Elegant Themes Divi theme, whose developer makes claims about RTL support on its pre-sale pages.
I've used themes (like Weaver II - another Swiss knife type theme) that simply work out of the box with RTL, but now I see that this isn't the case with Divi. There are complications; and their forum is full of attempts to fix things with custom CSS. That's not how it should be.
I can't prove it, but my hunch is that themes that start with RTL built in from the ground up, from when the theme's coding began, probably work better than those that add it later as an after-thought.
In any case, a little honesty is called for from web developers. It's better to be up front about what a theme supports rather than deal with disappointed users later. Sometimes a user may be willing to get his/her hands dirty and try to deal with short comings simply because a theme is, in many ways a good solution. And user needs, just like themes, come in different shades. For example, it seems that the Divi theme works more easily in an environment where only RTL is needed, but may be more complicated in a case where a multilingual site is needed. (Now I'll have to see.) Or, a user may require only slight and occasional use of right-to-left languages. I happen to need the theme for multilingual sites with hundreds of articles in Hebrew, Arabic but also LTR languages. I previously have extensively used SPIP CMS, which works very successfully with multiple languages. SPIP got it right. I don't think it's rocket science, just a matter of dedication, on the part of developers.
Update on use of the Divi theme
On the small site on which I've been working, the main RTL problems have been resolved by quick responses from Divi's tech support. Every issue required a separate custom CSS fix. For all I know, there may be dozens of further possible RTL issues.
The fact remains that some WordPress themes get it right from the beginning; I'm not sure how. In the case of Divi, it seems to make better economic sense for the developers to rely on their tech support than to ensure proper RTL compliance from the base up.