I used to be on Wordpress for the longest time, but I decided to move to Ghost. Here's why.

Open source.

I like that it's all open source and still profitable; I think the team is doing great work and I'd like to see more of this work in the world. This leads directly to the next point:

Very low cost to get started, with lots of features.

Ghost can be free except for your elbow grease, server costs, Stripe payment costs and domain name costs. Beyond that, you just need to update it yourself; there's no additional costs for the software itself or any commission. This includes SEO and other important features that help you with running your site. For Wordpress, the jump from "Free" to "Paid" is more steep and with fewer features.

Great learning project.

Ghost is a good starting place for people who want to understand how a site works roughly, and the process required to set up a realistic one. It could be pretty complicated looking at first, but once you get the hang of it, it helps you understand web development from start to finish.

Better than alternatives.

Ghost also has a pretty excellent write-up on their own:

Ghost Alternatives – See how Ghost compares to the rest

Considering switching to Ghost? You’re probably trying to figure out whether it will be the right decision for you. Find out how Ghost compares to others.

Ghost - The Professional Publishing Platform

What are the drawbacks?

Lack of marketing.

You'd need to figure this out yourself; it's not as easy as sharing a community among Wordpress bloggers, for example.

Subscription inertia.

It's hard to get people to "subscribe" if it looks a bit different than a mailing list and to click on a magic link (though that can be fixed with some twiddling).

There is a learning curve.

It's definitely not as easy as setting up Wordpress (free version) if you want to do it yourself. It requires some knowledge of servers such as how to access them via ssh and networking to get the domain working.

No fully-free version.

There are still some costs incurred in running a Ghost blog like this one. This means that if I stop paying the costs of running this server, my blog will go down. I can't revert to a "free" version if something happens.

Conclusion

Despite the drawbacks, I still chose to use Ghost because the benefits outweigh the drawbacks for myself, and it was time for me to upgrade to a different hosting server.