I briefly dabbled with an LG G5...
But quickly came to the conclusion that the mods weren't worth the hassle.
Admittedly, this was partly because you had to switch the phone off to switch between them, but the key reason was that I couldn't put a case on it while a mod was attached. And thanks to the educational medium of bitter experience, I have a standing policy of always having a case and screen protector on my phones.
Motorola's implementation was a lot more sane, and a lot of the mods effectively acted as a case, but that'd mean keeping the mod permanently attached, which wouldn't be ideal for things like the camera mod.
Equally, scanning through the list of available mods (https://www.androidcentral.com/moto-mods), there doesn't look to be that much of interest apart from the Hasselblad True Zoom - barring the projector mod, they're pretty much all variations on speakers and battery packs.
And therein lies the issue - for the same price as a given mod, you can generally buy a standalone equivalent, either for less money or with better features. And as an added bonus, the standalone item will be usable with other devices.
E.g. the projector mod is listed at £200 on Amazon. And on the very same page, there's a pico projector advertised for the same price, which runs Android, has a built in battery and claims to throw up to 130 inches, or nearly double the Projector Mod's 70 inches.
And I'd still be able to use my phone while streaming media, to boot.
Admittedly, the value proposition is a bit different now that the mods are being cleared out, but even so, you're buying into a dying eco-system, and while the Z3 is still a pretty shiny piece of kit, you'll probably have to throw the mods away if/when you decide to upgrade in a year or two.