I partly agree what is discussed there.

Currently when adding properties to pages in order to include semantic annotiations is really hard for normal users in general, SMW+ tried to counterattack this, but really did not achieve it in normal user point of view.

I have implemented several SMW:s to my customers, and the love it (as end product). however, they are not interested in editing pages as raw syntax but through forms. This is really the downside of SMW. 

for me syntax is clear, but for end users, even remembering that [[Category:Some Category]] and [[Property name::Property value]] has difference, one or two :, is a problem.

SMW is great, I like it a lot, but end users in general might not be aware (and should not be) what happens beneath.

Even mediawiki syntax itself may be too technical for some users.

About perfomance, SMW annotiations in pages do not cause pages to load slowly. Only noticeable difference is when saving the page.

It is true that poorly designed structure of annotiations, will cause problems, especially when there is lot of relations...however, I'd like to say that sql db design has similar problems,
SMW requires also very deep knownledge in order to be efficient, so does DB design of softwares