I have used many parts from Huztl/ Farmer Tec as well. So far I am happy with them, and my customers are too. They like the fact that I can save them money on repairs over the cost of going genuine on big ticket items, like piston and cylinder kits.
Specific examples: Piston and cylinder for my MS440 as well as a recoil assembly. Recoil was metal and works great. Piston and cylinder still going strong, no issues with power. Not sure how many tanks of fuel but quite a few. 20-30 tanks if I had to guess.
MS200T piston and cylinder, carb, fuel line and filter. Still doing great, probably 20 tanks of fuel through it now.
MS250 carb, 029 carb, 044 oil pump and worm, M5 screws, 025 coils, 066 rear plastic cover...
Have used sprockets and clutches from them with no issues. Crankshaft for my MS440 project with bearings and seals doing fine. Piston and cylinder for 029 running great. Worms and oil pumps doing well.
I guess I'm not much help if I don't have any complaints, huh? I honestly have had little trouble from aftermarket parts as of yet. Yes, fitment sometimes is a little off, but rarely to the point that it adds a lot of time to the repair. If it doesn't fit, it doesn't get installed on my customer's saws. I have gone genuine due to fitment before. If that air filter doesn't fit, it's going to dust the engine. I can't have that on my customer's saws. I have started going genuine Stihl on air filters lately. MS170/180 not so much since it's just a piece of felt, but the others have to fit right, and they don't always seem to. That's the only complaint I can think of.
One more comment - The thing I like about the aftermarket carbs is that they have all three adjustment needles on them. I can adjust the carbs when I need to without dealing with the limiter caps. Call me crazy, but I much prefer the old style spring and needles.