I have done some Sharepoint development but I mainly specialize in PHP/CMS/LMS solutions and yes a little Wordpress.
First whether or not Sharepoint is a good solution, has a good API, is fun to code, or whatever else is irrelevant. The fact is that there are hundreds/thousands of companies using Sharepoint and many of these would love to have a good Sharepoint developer.
I worked with someone who turned a basic Sharepoint site into a fully functioning (with tons of bells and whistles) employee tracking and PM tool. This saved the company well over $1M in 3 years and worked better than most solutions I have seen.
The fact is that most developers are negative about the Sharepoint API (which is lacking and quirky) only strengthens your employment opportunities. There is nothing wrong with coding niche systems at all.
I have found that if a company goes this route and has a great developer they never stop using that crappy platform. But if they have poor developers they will move to something else quickly.
So if you are good you will have long-term employment if you want to keep doing that - maybe really long-term. I don't know how many developers my company has doing legacy stuff. These guys who wrote code in the late 90s and still supporting it. Yes eventually their stuff will be replaced. But guess what? They will be needed during the switch and quite possibly will know new code better than anyone.
My advise is that there is nothing at all wrong with being a Sharepoint developer. However please allot a good amount of time staying current with the industry/methodology that you want to be a part of in the future. Often doing this will allow you to take newer methodologies and implement them into sometimes older/not-quite-as-good APIs.