I can tell you that I've been involved in the design of piping for my entire career of over a couple decades, and I had to look up what a "sweep-o-let" was because I had never seen one before!
I can tell you that they are rare indeed for linesizes below 8", which is where I've always lived.
Despite never having seen one before, I understood their advantages- and disadvantages- instantly when I saw the part on Bonney Forge's website. They are an entrance loss, stress intensificaton/fatigue failure mitigation tool, and a fine alternative to the other solution to such problems, which below 8" is typically the installation of a reducing tee, requiring new butt welds in the run of the line. The use of a reducing tee becomes very rapidly unfavourable as line-size increases. This device reduces not just the cost of the tee, but also eliminates the two full circ butt welds to a much smaller single butt weld, albeit one which could be difficult to examine by radiography I would imagine.
Whenever stress intensification, fatigue/thermal cycling or entrance losses are not a primary concern, people are going to use either another type of o-let (butt weld-o-let, sock-o-let etc.) or a stub-in connection because the cost of the components and welding will be lower.