I can definitely speak to that (having done multiple go-lives for third-party ERPs and WMSes and such for my current employer). There are definitely tradeoffs between going with something off-the-shelf v. developing in-house v. something in-between (e.g. a system that's ostensibly "off-the-shelf" but expected to be heavily customized), and the vast majority of the time you're gonna be best served by something closer to the "off-the-shelf" end of that spectrum, no matter how much you think your "special" business needs warrant it.

That said, a lot of these sorts of systems are notorious for being pretty awful, too. Yes, there are many millions of man-hours poured into them, but that doesn't mean they're good; those man-hours could very well be spent on trying to implement every bell and whistle under the sun while entirely neglecting bug fixes, for example (and - speaking from experience - that is a disturbingly-common example). Also, having a bad UX can (and often is) a fatal problem for such a system (sometimes even in a tragically-literal sense, e.g. with EMR/EHR systems and other medical software); the whole point of business software is to optimize user productivity, so the last thing you want is to make the user's life harder.

So yeah, tradeoffs. If you're gonna fall into the DIY trap, the key would be to not try to replace the entire ERP or entire WMS in one go, but rather to build smaller tools that optimize specific business processes, and make sure those tools stay small and maintainable; any expansion of features should happen through interoperability between tools rather than trying to build a giant bloated tool. If you're thinking "wow, that sounds like the Unix philosophy" and/or "wow, that sounds like microservices", that thought ain't exactly a coincidence.