I’ll gladly explain.
Back when I competed previously, I began competing conventional and got seriously stuck at 405. At the time I had no clue why and kept busting my head against it to no avail. So I started dabbling in sumo. Some time later at a meet, Steve Scialpi offered to work on my form which frankly sucked. He got me a PR that day and I went to train at his gym for a year. Conversion to sumo was now full speed ahead. While the sumo was increasing every now and then I would pull conventional but was never able to feel comfortable or get stronger in the conventional style.
Now after a long absence, I’m back and though I’m using sumo occasionally (conjugate), I’m primarily pulling conventional and I feel stronger and more comfortable pulling conventional. I believe that when I was pulling mostly sumo, my back was not getting enough attention and began to lag eventually stalling my sumo shortly before I left the sport.
Regarding the recommendation for beginners, I feel that beginning sumo too early will handicap back development, and overstress the connective tissues of the hips and knees which are already being hammered by squatting. Also, the conventional technique is much easier to master than sumo and a beginner has enough to pay attention to at that stage.
Of course, like anything else, this is an opinion based on one person’s experience so the only way for anyone to be sure is to experiment, hopefully under a good coach’s eye.