I don't think that's really the case for most people, they're just responding to blanket statements condemning an entire well-respected company's line of products.
Castle's ESCs are very popular in the offroad world, but they never tried to answer the desire for a sensored motor before.
Tekin is also an excellent company, but it seems like these days people are more often just buying their motors for offroading and not their ESCs, which simply point to them possibly needing to also release a new product that fills a need.
I personally think the more competition the better, and Tekin would be smart to engineer and release a modern offroad-specific ESC with up-to-date features in a reasonable-sized waterproof housing.
Every company sells the occasional defective item, and when you bring soldering and programming into the mfg process there is more room for error.
A you have personally found, sometimes it can be ridiculously complicated to troubleshoot after it's connected to all the components of a RC vehicle.
There are so many variables to take into account, although I know you have eliminated what seems to be every one of them, with no luck.
I'd try to find someone you know that would install your particular MMX (s?) into another rig, reset everything and then try it out.
Castle, like all but the tiniest of companies, is not likely to test and report what, if anything, they find wrong with them when you return them.
Companies these days just send a replacement because that's the most cost-effective policy for them.
I do feel for you, man - and I'd rant, too - but I'd try to keep it in perspective.
Worst case scenario you should at least be able to return it (them?).
Peace...
