Neigong will give you the ability to work directly with your own bodies energy. You will cultivate significantly higher levels of energy that will enhance most other practices but there are a few that can create problems if you aren't careful.
Reiki and similar practices where you are guiding universal energy through your body and into other people/animals or objects does result in you draining your own energy, even if it does not seem that way. Those who practice reiki or something similar should take care and be aware of their own energy levels if they want to combine neigong with something like reiki.
When we use qi for healing, qi is emitted in short bursts, lasting 5-20 seconds normally and allows the body to replenish itself in-between working on patients. Reiki and other practices requires the flow of energy for much longer periods which can over time cause a draining effect and decrease health as a whole. It's just about being aware and pacing yourself.