I straight up admitted that the lack of a text tool was its one real shortcoming, and I agree that at least is a problem, but the rest of this is goofy, haha.
a.) Shape tools are something that really shouldn't even be used in the first place due to the generally artificial and out of place look they give, but there are multiple methods to drawing a perfect circle or a square in sai that honestly aren't difficult at all. Specifically the method of simply dropping a dot and erasing the inside, and using the curve tool both work well. Heck, I even found a third way looking for links to these examples, haha.
b.) After looking up the layer options in Manga Studio and Clip Studio Paint I'll straight up say they're unnecessarily complex, and you can get the same exact effects in Sai just using different methods and tools. Sure they might be nice if you're feeling lazy, but they aren't by any means offering you anything you couldn't achieve just using sai.
c.) Perspective lines take all of about two seconds to sketch out. Sai has a means of making lines with literally ever brush it has, and once you get right down to it all perspective is is a dot or two or three with a bunch of lines coming out of it. :/
d.) The magic wand tool is so adjustable you really can get it to do just about anything you want. You just have to take two seconds to actually look at the settings and adjust them to whatever it is you're doing.
Like, I'm not trying to start an argument or anything, I guess my point is that sai, while simple, works perfectly well for drawing and comic making. The additional tools found in other programs are just a lazy way of doing the exact same thing you can do in Sai, and, again, if you understand how to draw in the first place you probably won't need most of those tools. It's always astounding to me how many people act like a program not doing everything for them means it's somehow lacking in something, lmao.