Warranty? LOL, it's a transformer. I've a 50 year old tube radio and the transformer works fine, as does my 40 year old X-ray machine transformer. If a transformer goes bad, I think it won't wait many years.
Bryston is a pro-audio manufacturer. Size is definitely an issue, and I'm not surprised they use toroids. I wouldn't expect them to use R-core due to availability issues in North America.
R-core is not rare, it's just rare in Western nations; it's fairly common in Japan etc. One more advantage I forgot to mention before is its efficiency, it's more efficient than a toroidal, not to mention EI.
BTW, I often wind my own EI secondaries, though I'd never consider doing it for a toroidal, it's next to impossible to DIY any reasonable number of turns. R-core however is the easiest to wind of all, you simply spin the bobbins, since the core cross-section is round, whereas toroidal and EI all have rectangular core cross-sections.
If you're set on toroidals, I'd still recommend Victoria Magnetics. It's also Canadian like Plitron, I've seen good prices from them, and their workmanship is excellent.