I think most of the "quality" holster makers use both; giving the consumer options. Some only use belt loops though, for dual attachment rigs anyway. Milt Sparks use to make a Watch Six holster, which had dual clips, as did Lou Alessi....(who I am 99% certain designed it). You can see this design in UBG's Striker and in a couple of the models point out by Tim Thurner, like the KX Lightweight I have enroute for my P365, but Sparks doesn't make it anymore.
Now, I like, and have both clips and loops for IWB holsters. Granted, a one-way snap-on belt loop is more secure than the same holster using clips, but for the utmost security, you need to use belt slots, which basically means OWB. I wouldn't think of using a cheap holster with a clip on attachment, but I don't have a problem with using clips from a "name" maker, like TT, Sparks, UBG, etc. I also tend to think clips are okay with the lighter guns, but if I am going to IWB carry a Govt Model or Glock 17 or some other behemoth of a pistol, gimme belt loops all day.
Now, to Day One with wearing the Kusiak: I wore it from about 0600 this morning until just a bit ago, almost 9 hours total. I helped a friend who owns a nursery put plastic sheeting on some greenhouses, so I was bending, tugging, walking, lifting, stretching etc. all the while wearing the Kusiak with my P365 AIWB. It really is breaking in nicely. No, I didn't "almost forget I had a pistol on", I am very much aware of that thank you, but to answer the obvious questions: Yes, it is quite comfortable. It also stayed in place, like it's supposed to do. And it held my gun securely. The holster does allow a smooth draw, but you have to come straight up and out, no dragging your gun out of this thing, it won't let you pull. So, for like $47 bucks, I got a nice, thin leather IWB Holster that I think is going to serve me very well over the years. I still maintain that the inbound TT Gunleather KX rig I ordered will be the better holster for all day wear, but this Kusiak is primarily going to be my quick and easy clip it on/clip it off holster. I'd recommend one for any small, lighter weight pistol or revolver; up to and including something along the lines of a Glock 26 or S&W M&P Compact. Anything larger, for me anyway, is going to call for a dual BELT LOOP attachment , like a TT Mikes Special, Sparks VM II, or a UBG Canute.