Certainly because TVHeadend as a long history with LibreELEC the biggest benefit might be the support from other TVHeadend users here. However being multi-plaftorm most questions you have will not be LE specific and so you can probably be helped best on the NextPVR forum. Being a NextPVR user since 2006 I am certainly biased but the simplicity of NextPVR doesn't mean that it is any less powerful or stable then TVHeadend.
The biggest weakness I can see in NextPVR would be no bouquet support and support for multi-dec, CAM/CI and softcam would likely be via minisatip. I am in North America and I prefer the support for capture devices like the HDPVR that NextPVR gives me.
Regarding closed source I have also developed several client and utilities for NextPVR over the years and sub has always been able to help me with API calls. In the next version there are improvements to tighten Kodi and NextPVR integration so it is a vibrant project.
NextPVR isn't just a backend though, there are many clients other than Kodi. Client platform has always been a strong point. My username in fact comes from my client work for the Hauppauge MediaMVP. Most users here no doubt like Kodi and while I am the lead contributor for pvr.nextpvr I probably use the native uidroid client on Android more these days for quick PVR use.
The C# vs C/C++ debate can go in both directions, and clunky is not a word I would choose. NextPVR server does use native c++ code for the interface to the DVB subsystem which will be the core of both NextPVR and TVHeadend for digital tuners. C# has the advantage of multiplatform components that don't have have to be custom code, and Microsoft regularly updates the core with enhancements and security patches. I have not heard any complaint about Emby or Jellyfin simply because the are netcore 6 like NextPVR.
Bottom line if you try NextPVR and find something you don't like you can probably get the issue resolved here or on the NextPVR forum. It is worth a look.