I use to use a light weight extension in my show geldings tail. It was a 1/4 lb tail. He always has a beautiful long tail, but wouldn't fill out at the bottom, so my trainer at the time made him this 1/4 lb one, and it NEVER looked fake. It made his tail just look full and gorgeous. I think the fake look comes from improperly putting them in and having more fake tail than natural tail. It shouldn't be a replacement for a tail.
I have never used a forlock extension, but I can see where those would look really fake.