Spark is really great! I used it on iOS for a few years and loved it.
A few months ago I decided to stop using it, though, for paranoid reasons: I couldn't quite understand how the team behind it could afford to invest such significant development resources in a free app.
Spark does offer some paid plans for "team" features, but I don't know how many customers they have or whether they're profitable. Since my email account is the single most important account I control, I felt more and more weird about Spark's servers having full access to it when I didn't feel fully confident in their business model.
That said, I've never had any actual reason to mistrust Spark or doubt the quality of the product. It's probably the best email client out there today. 🙂