I heard the same story. So I bought a genuine NL filter from an official NL dealer, and two Motorcraft filters that would fit. Why Motorcraft? Because I put on a Ford Motorsports 90 deg filter adapter to eliminate the mess of changing the sideways filter on the NL 6KW Shibaura engine, and it uses Motorcraft filters. I was told this is horrible as I now can't use the NL filter.
Anyway, I cut the NL and two Motorcraft filters apart and inspected and measured everything (and I have pictures to prove it). Both Motorcraft were somewhat higher quality than the NL. The relief valve is set to about the same pressure. The NL has a bottom relief valve which is the cheap version, the Motorcraft both had top relief valves, considered to be a lot better. The anti-drainback was silicone and higher quality on the Ford. Both Ford filters have substantially more filter area.
There is merit to the argument that gensets are a little different, as they start at full throttle/full rpm which you do not ordinarily do, and that could open the pressure relief. And I can understand NL and representatives of NL recommending their own filter - they know it and love it and make a profit from it. However after looking inside, I've got no regrets using the Motorcraft filter. I don't really care that much about the price (though the Motorcraft are about 1/3 the NL), but availability is much better, and not having to clean up the cup of oil spilt by the sideways filter is pure gold. That said there are certainly cheap filters out there that I would not use on anything. A filter cutter reveals a lot of sins.