I know there are quite a few powerlifters who follow high frequency routines as well, but I also noticed that some of the best only hit each major lift once per week and also had a bodybuilder-style routine as well.
One problem I had with the upper-lower split was that after I reached high bar squats for 18 reps or so, I could not tolerate two lower body days per week anymore, at least not with the routine I was using.
I also started to think of a few things. If muscle growth becomes so damn slow after some time, does it even matter if you train a muscle less frequently, but not so infrequently that one will regress between workouts?
And with all this talk about protein synthesis, it's not like so much damn protein synthesis is going on between workouts after some time. I know that Dante (who I like) and Jason Ferrugia and some others frequently talk about "52 growth cycles per year versus 75 to 105 growth cycles per year" but I am positive it is not that simple and it doesn't work like that.
A lot of gurus also contradict themselves. They say on one hand that just getting big and strong on the compound lifts is nearly all one has to do and that the compound lifts hit several muscles per lift (which they do) but also say you have to hit each muscle twice per week. Well this is done even if someone is a bro split! If I hit chest and biceps on Monday, I am working my shoulders, chest, and triceps, then I am also working chest, shoulders, triceps when I have my workout for shoulders and triceps! If I hit biceps on Monday, and then later hit back later in the week, I am working my biceps twice per week. If I hit back and then days later on leg day do stiff-legged deadlifts, then my back is being stimulated twice per week also!
Also, if enough volume is done, it's not like the body regresses in five to seven days. So even if less frequency is less optimal in some respects, one will still have desired results with consistency and time. That's my take on it.
I do three to four exercises per muscle group for three to four straight sets. I warm up and use the same weight for all sets but go short of failure or if it's failure it's just a difficult set, but I am not convulsing, grinding and screaming. Failure is inevitable though if someone is working hard. It's hard to hold back sometimes.
Also I found that just tacking on an isolation exercise to upper-lower splits for weak body parts (size wise) was not doing the trick.
Edit: @La_Crosse_Grad read this post.