in my experience this brake works fine, even on a relatively heavy machine like a Trident. In the end it all depends on the correct setup of the brake.
In my experience - now 33 years with at least one T160 and four years with a T150 with the earlier drum brake - the drum brake 'works fine' if you don't use much of the triple's performance, don't carry a passenger or much luggage and stay away from twisty roads if they also go downhill for many miles. And, as I say, this is with a pre-conical drum that has had much collective experience lavished on it - drum checked for ovality, matched linings, top-quality cable, yadda, yadda.
Otoh, any half-competent diy mechanic can improve the disc brake - single to double, floating and/or 12" discs, four-pot calipers, etc., etc. - until it's other cycle parts that invoke the law of diminishing returns.
Bottom line is even a kick-start triple is a near-500lb motorcycle when filled with fuel and oil. Fitting them in 1968 with standard cycle parts also used on the contemporary twins was simply short-sighted - the CB750 always had a disc, even though Honda made some very good tls drums; the GT750 had a 4ls for just a year before Suzuki acknowledged the inevitable, etc., etc. Then quite how BSA arrived at the collective conclusion that what their top-of-the-range models really needed was the drum brakes replaced by different drum brakes of the same sizes is likely to remain a Marie Celeste-type mystery, I suspect.
These brakes can be upgraded by fitting 1-inch longer brake lever arms,
... at the expense of longer handlebar lever travel.
Don't get me wrong - if it's what you want, either BSA/Triumph drum brake can give reasonable performance in modern traffic conditions; just, for similar effort and expense, even the '73-on Triumph/Lockheed disc will always reward with greater performance. Also, once disc brakes are done, their performance will deteriorate almost unnoticeably even if never maintained; otoh, maintaining peak performance with either (any?) drum brake requires regular fiddling and fettling.