It should work for Raileasy/TrainSplit.
Of course, selecting a third-party retailer is made quite difficult (particularly on mobile), you have to resist clicking the big yellow Buy Tickets button or you'll be sent to the flow's TOC.
Yeah it varies by retailer.
But for example with Raileasy it is better but still not perfect. If I search for Chester to Hawarden Bridge leaving now and on the NRE website I specify an additional 120 minutes for connections (I know, it's a bit contrived, but NRE also limit how much additional connection time you can add), the search results on Raileasy don't reflect that additional connection time. I'm not sure if that's NRE's fault or Raileasy's mind.
It's then also a bit odd because you'll be shown a price on the NRE site, then handover to a retailer like Raileasy who offer split tickets with a great split ticketing algorithm, and the fares will be less and the journeys aren't (I think) automatically selected).
Raileasy/TrainSplit are certainly better but I don't think any retailer has a perfect handover from NRE.
As an aside, why doesn't the forum's website appear in the list of retailers to buy tickets from NRE? I assumed pretty much any accredited retailer would be on the list! I've never noticed before because I stopped using NRE for booking tickets years ago.