It's not that client-side applications are easier to reverse-engineer. If the application is all client-side, it can literally just be downloaded and reused by a would-be competitor.
Microsoft Word is entirely client-side so it too can literally just be downloaded and reused by a would-be competitor.
(If I were in a nitpicking mood I would also point out that the source is what is fed into the minifier or obfuscator, not what comes out.)
It's a lot easier to reverse-engineer the site than to build it from scratch yourself. You won't get all the comments, and some of the names of functions and variables and whatnot may lose their semantic meaning in favor of shorter names. But yes, it is trivial to unminify the source and work through it. If it's 10k+ lines of code, that's never trivial to understand and get it all in your head, but the concern is absolutely valid. If you think minifying/obfuscating is securing your code intellectual property, you're fooling yourself. It's like security through obscurity... it's not really offering you any protection.
I'm a curious guy and have deciphered small-ish amounts of minified and/or obfuscated JS before. It is absolutely not trivial. Especially if it's obfuscated.
"un-minifying or un-obfuscating JavaScript is not a trivial exercise by any means."
I think you mean that understanding obfuscated code is not trivial. Un-minifying is certainly trivial. There are tools to do it for you. There's no such thing as un-obfuscating, if you consider that you can't get the original comments or all the semantic naming back. So I'll concede that it's more difficult to read through this obfuscated code than the original. But the original comment in this long thread seems valid to me... that an entirely client side app puts you at much greater risk for cloning than an app with a significant portion of code in back-end, just because your javascript is out there for all to see.
My point is that you can't just de-obfuscate a primarily client-side JS app and replace the Firebase (or whatever) endpoints. Proprietary obfuscation is more than just variable name munging and minification..