WYSIWYG HTML editors disadvantages.

What is a WYSIWYG HTML editor?
WYSIWYG (don't hurt yourself pronouncing it) stands for What You See Is What You Get. WYSIWYG HTML editors let you create web pages (or HTML pages, if you like) without knowing HTML tags (or even what HTML stands for :-).

The Disadvantages

As I mentioned before, WYSIWYG HTML editors are fine for beginners or if you want to create a web site quickly and don't want to fiddle around with HTML code. However, if you plan to develop web sites for a living or you want serious webmaster(website owner) for your business you need to learn some basik HTML and it's very easy.

I don't use WYSIWYG HTML editors; never have. That's because I belong to an old school and also there were no editors available when I first learn HTML. But I don't look at these programs disdainfully. In fact, the disadvantages and the flaws I am about to mention have more to do with showing you the light and helping you become a better web content writer/editor.

Web pages written using WYSIWYG editors are rigid. They do not stretch if you put more text or images on a page. Instead they overlap on each other. Some WYSIWYG editors produce excessive div tags. It is difficult to edit these pages in an ordinary HTML editor.
A lot of HTML code produce by the editors is superfluous which makes the web pages bulky in file size and as you will know, a large file downloads slowly that a smaller one. Also search engines need to wade through all this code to get to the meat of the page - the actual text contents.
WYSIWYG editors let you ignore certain aspects that are important to say the least. For instance, the need for the image ALT attribute should be explicitly mentioned by the editors and images should not be included without the ALT text. If the purpose of a WYSIWYG editor is to hold a beginner's hand and guide him/her through the process of creating a web page, the programs should tell the importance of certain HTML attributes and tags.
The ALT text is used by search engines and thus it's a good idea to have your chosen keywords appear there.
Another example is the heading tags of a web page. These are very important from a search engine's point of view. Thus, you need to put your keywords in an ‹h1› or ‹h2› tag and not just have text that "look" big and appear as a heading.
Several editors have been accused of letting users employ "browser specific HTML tags". These tags, as their name suggest, will appear well on certain web browser and ignored by others. These tags are also generally not compliant with the HTML standards set by the W3C (World Wide Web Consortium).
Most editors still use HTML tables to layout a page. This is an older technique replaced by Cascading Style Sheets.
HTML written by the WYSIWYG editors is usually not correct or validated. I am not saying that all web pages need to have valid HTML - in fact the web pages of some of the most popular web sites in the world would not validate if you checked it out. But as I mentioned above, if you are plan to make a career in web development you should know that your HTML code needs to be validated and not just look pretty in a web browser.
HTML was meant for document structure not document design. However, WYSIWYG HTML editors work on exactly the opposite line of thought. One needs to understand that HTML determines the different elements on a page such as a heading, paragraph, list etc. How these elements will look is the prerogative of the web browser. Also, with Cascading Style Sheets (CSS), the design and "visual" of a web page is segregated from the actual content. Thus, the content of the web page resides in HTML while the look (colors, positioning etc.) lies in the CSS rules.
WYSIWYG HTML editors might help beginners create web pages quickly but an experienced web content writer equally fast (if not faster). Further more, pages developed by experts are optimized and adhere to standards.
WYSIWYG editors simply do not provide enough support and help in optimizing your web site for search engines. This is vital! You can have the most beautiful looking web site in the world but if no one's able to locate it... well! And this is what we will discuss next.
Most important side effect is less content more HTML code produce by WYSIWYG editor which will be penalize by search engine.
Density of your keyword will reduce automatically by WYSIWYG editor which heart your ranking on Search result.

Search engines and WYSIWYG HTML editors

The question that should be of interest to all users of these editors: Do manually coded sites rank better than those developed with WYSIWYG HTML editors?

Not necessarily. Search engine algorithms take several factors into consideration and some of the important ones are:

Web site content: Content needs to be fresh, meaningful and useful Number of inbound links: The more the number of links your site has from other sites the better. Quality of these inbound links is also a factor. Thus, incoming links from famous sites carry a higher weightage than those from less popular web sites.
Presence of keywords at appropriate places on a web page: The mere presence of keyword does not help. Keywords need to appear on the so called "hot spots" like the TITLE tag, the heading tags, link text etc.

Though a web site developed in a WYSIWYG HTML editor might rank higher than ones coded manually (if one has followed the important Search Engine Optimization principles), I suggest that you keep the design and layout simple. Do not create complex designed web site using WYSIWYG HTML editors. Because the more complex your design, the more the superfluous code generated by the editor. And this makes it difficult for search engines to dig out the "actual" content of the web page. In my opinion, keep your designs and layouts simple and clean.