Have become a fan of tarot. I use their gears, their metal is good! And machining is amazing!
Like tony said. You want be away from HK shafts, bearings and gears. The frame.. It'll hold perfectly fine!
The heads, are good, but sometimes the metal is softer aluminum, which won't make that big of a difference, I mean, in a crash metal will give, and you'll need to replace it.
But, sometimes the metal parts are over machined, making the bearings not a perfect fit, and slop in different parts. Hobby shops hate them, cause its obvious , they sell trex stuff and it's their bread and butter.
Thing I was frustrated with was the screw quality of HK kits, but that was because I believed I needed to TIGHTEN them , and didn't have a good solid screwdriver set, so screws would strip, I would curse things and 10minute 5 dollar repair would become a 30 dollar 3hr repair. I realized, you just need to snug the screws and let loctite do it's job, and give loctite couple of hours to set before maidening it.
Since that realization, honestly in my align 450 bird, my tail bearings assembly is HK, frame is HK, motor mount and main shaft beating holder is HK. Sometimes I throw a tail boom that's HK or exi. But I now get tarot or align stuff so HK and exi doesn't happen that often. My head is tarot, FBL os tarot... Canopy os tarot.. Lol
It's trex by name.. But my bearings are all align or boca or this amazingly cheap and awesome bearings shop (us based) rc-bearings.com. Man that guy saves me the bucks. (os engine bearings 30 dollar a peice.. I got from him for 6bucks, and they are all made in us. )
So substitutions and mixing and matching is what I am a fan of.

until and unless they are perfectly similar.