I think couplers hold up. I haven't used them, but the tapered teeth seem conducive to a tight joint, and (I think) will accommodate any wear over time. But no experience, so others could tell you more.
Coupler bikes were a fantastic innovation when other options didn't exist for a high quality compact bike (i.e., not a Raleigh 20, strong but heavy and limited gear range). But coupler bikes still have the drawbacks of multiple "loose" parts. But again, they were a lifesaver back in the day, and still are for someone that wants full-size wheels for any reason. Having said that...
Had high quality 20" wheel folders existed, like a Bike Friday, there would have been a smaller market for coupler frames. Regarding your BF:
Weight is most critical for rider weight, in the middle of the frame. As I mentioned previously, rear rack weight will be transmitted directly through the rear dropouts to the rear axle and wheel, and the rest of the frame shouldn't feel that weight. But if you want to run a Travoy, that's also an option, though you should hook it up with touring weight and do a test ride, including fast descents, to make sure of no stability problems; I used a B.O.B. trailer on a three week tour, which had a pivot behind the rear tire, like a car trailer and not a 5th wheel, and on high speed descents, I had to really lock the steering and make slow movements, otherwise I would get "sway amplification", stability issue, and this was with a LONG wheelbase recumbent; On my 700c race bike, in a short slow test ride, it was uncontrollable. The Travoy attaches at the seatpost I think, better than behind the tire, but you should still check stability with a load, including climbing out of the saddle, rocking the bike left and right, the high attachment point may be an issue, or not. Most trailers, with the trailer hitch pivot at the rear axle (like a 5th wheel geometry), should have little to no problems in this regard. However, traveling, the Travoy may add too much transport bulk, maybe not if treated like panniers all strapped together, so a big waterproof duffle strapped to the Travoy.
Amtrak issues, as noted above, might be an issue with a coupler bike, though if in a bag (a must, due to loose parts), I would bet you can bring onboard and put in the luggage rack with no problems.
You could also just "Rinko" one of your current large-wheel bikes (japanese bike touring train style; wheels off, handlebars/stem removed, fork removed, and into a bag with frame). This is not fast disassembly or assembly, but would work for the beginning and end of a big trip. Possibly would work for Amtrak in-car luggage. Definitely would work for air travel where bikes are not charged extra, but you may pay normal checked bag fee. Southwest airlines where checked bags are free, a big NO, a rinko'ed bike won't fit in their 62" max girth, and for oversize Southwest charges a lot.
My personal feelings summary: For light touring, take the Bike Friday NWT, if it's a comfortable fit. For heavy touring, I think you could still do the NWT, and have a lot less transport bulk without the Travoy. But I cannot recommend the original BF rear rack if you have one, crazyguyonabike (big blog online, world tourer) used a NWT with one and the folding rack fell apart, I think you need a better one. BF NWT should also do air travel within 62" max girth, according to him (web page below).
Relevant: When Ewan McGregor (yes, that one) and Charley Boorman decided to ride motorcycles in heavy touring around the world, they first contacted a respected dirt-bike maker in Europe, who hedged at the loaded weight and mileage. They then spoke to BMW about using R1150GS bikes, one of the first "dual sport" motorbikes designed for both on and off road, and BMW said, "Of course. It's what it is designed for." My Point: Traveling with a bike? It's what the Bike Friday was designed for. Frequent folder, folding and unfolding every day? Brompton; It's what it is designed for.
Or if you need things like a suspension fork, and if few times needed to break down the bike, go Rinko with your full-size bike.
Useful links:
Rinko:
https://www.bantambicycles.com/blogg...ko-vs-couplers
https://www.rossmancycles.com/rinko
https://www.welovecycling.com/wide/2...ing-your-mind/
BF NWT touring guy (both trailered and not, huge tours, third world, TRUST his durability comments, this is just one page of many on his blog):
https://www.crazyguyonabike.com/doc/...page_id=367799