I suspect we all have had unpleasant experiences with various vendors. In my case, it's mostly been with Moss.
Also worth noting that Charles Runyan, owner of TRF, is a genuine Triumph enthusiast. He owns several of them (including a very rare LHD TR5 and an even rarer "TRS"), and uses the same parts he sells (as much as possible) when restoring them. Last time I saw him in person, he had driven his personal TR3A from PA to CA for VTR/Triumphest. Also a family business; looks like his son Albert is slowly taking over more and more of the day-to-day business.
To the owners of Moss, we are just a "profit center" and not a very good one at that. We once had a Moss rep speak at a club meeting, where he explained that they weren't making enough money on MG and Triumph, so were going to branch out into Miatas and something else (I forget what).
I bought a front apron from VB a few years back (everyone else was out of stock). It was so poorly made that I would have had to take a cutting torch to it, just to push it into place on the car! Someday I might try cutting it apart and welding it back together in an attempt to make it fit; but I sure wasn't happy. It also wasn't complete, the turn signal holes were not finished to size and there were no captive nuts.
I'll add Ted Schumacher (TSi) and BFE (British Frame and Engine) to the list of performance suppliers. I've not dealt with the new owner of BFE (Brian Howlett) yet, but I hear he's a good guy. Ken (the founder and former owner) only sold out because he was in poor health (and has since passed away).
https://www.tsimportedautomotive.com/
https://www.britishframeandengine.com/
IF Joe ever gets his website back together, I would also heartily recommend Joe Alexander, of Alexander Racing Enterprises.
https://www.gasketinnovations.com/
ARE is also a family business, all 3 of Joe's sons are involved both in racing Triumphs and in ARE.
BTW, Joe is being inducted into the British Sports Car Hall of Fame in a couple of weeks, a well-deserved honor.
Don't forget, though, they sell mostly to the vintage racing crowd. A part that works well on the track may not work so well on the street. In addition to potentially short life, they may also shorten the life of other parts.
This is kind of off-topic, but I wanted to share this photo of two of Joe's sons, at the Kastner Cup race last year. Yes, that white car is actually a TR4, wearing a vintage fiberglass body known as an Ambro.