The RCGF 15 has two versions- standoff mount and beam mount. The earlier standoff mount version has a terrible reputation. The beam mount seems to be a good bit better along with more powerful too. So check which one you have.
That said, if you are concerned about engine reliability, make your second plane a .60 size sport model (Ugly Stick, 4 Star, or similar) and run each of those engines on it for 15-20 flights. If you have reliability problems you'll find them, and will do so on a plane that is easy to land dead stick.