I think your customers are running an outdated, or awfully developed Joomla website. Probably both.
I know my way around Joomla, and been using it since version 1.5, which was not all that long since the team behind Joomla moved away from Mambo.
I can tell you that the evolution to 3.8 is nothing short of pretty darn amazing. And I’m itching for Joomla 4 which will be a native Bootstrap 4 and PHP7 CMS. (J3.8 is already PHP7 compatible, but works on top of 5.6. And its Bootstrap 2 even if you can run Bootstrap 3 with the right framework).
As for WordPress, whenever I work with a WordPress website, I hate how convoluted the templates/pages design/build are. Widgets everywhere and whatnot. Visual this, visual that… fk me…
Joomla is way way way way cleaner.
Where Joomla is currently losing is with the Admin UI, from the POV of a regular user. Something J4 is addressing as they had to give in to the reality that to speed up adoption by website owners, Joomla admin board needs to impress the website owner, instead of being easy for developers.
Another thing WordPress did better than all the others, was their approach to their demographics, they did it well, they marketed it well, and now they just live of their mass adoption that keeps bringing them people while other CMS’s have to actually fight for a share.
But I grant you that the biggest problem with any CMS is the compromised plugins/etc. And even if that was not enough of a security risk, there’s no shortage of people installing nulled plugins/templates/etc to save a few bucks.