The smaller Generacs, e.g. the 5500, don't have a great reputation, but there isn't much to be "concerned about" when you already own one. Small oil leaks are a fairly common complaint, so keep an eye on the oil level dipstick. Most owners don't bother to have the leak fixed (it's not cheap), so just add oil as needed. Other than that, routine engine maintenance (oil change, filters and spark plugs) and regular exercise (run the genset under load at least monthly), same as for an Onan.
The similar size Onans are known for finicky fuel systems, so having an Onan might not have been any advantage.