In general, engine vibration isolation is good thing - if it is done properly. The trick is how to do it properly.

To put simply, the end goal should be to attenuate high-frequency low-amplitude vibration, which is detremental to the on-board electronics and airframe glue joints, and at the same time not allowing lower-frequency high-amplitude vibration to be introduced. Many of the "universal" isolation mounts do little of the former, while introducing a whole lot of the latter. That is worse than not using the soft mounts at all.

Broadly speaking, the best chance of achieving the design goals above is to emply isolation mounts, whose mounting points are spread as far apart as possible. This is not always possible with a pre-canned "universal" mount package, as firewall dimensions vary widely. Your best chance of a good mount is to copy Fuji's isolation mount design, but spread the mounting points as wide apart as your fuselage will accomodate.

Realistically though, the simplest and most effective solution is to hard mount the engine and let the airframe do the vibration attenuation. Fast moving air flowing over the aircraft has a natural tendency to attenuate vibration - but only when the aircraft is flying. So limit the amount of time the engine runs on the aircraft on the ground, and for cristsake, break-in your engine on a stand.