It's probably worth pointing out that every online payment service I've ever encountered has incredibly one-sided terms that allow them to screw the merchant more-or-less at will. Even if the service itself doesn't feel the need to use them, the card networks typically require them, so any service backed by card payments will almost certainly have them.

PayPal is notorious for this kind of thing, obviously to anyone who's been around relevant online forums for a while, but then PayPal is also the 800lb gorilla of the online payments industry so it's hard to assess how representative the nightmare stories really are. I don't know enough about the parent poster's story to offer an opinion, but I've seen plenty of vitriol directed toward PayPal from other merchants whose accounts were frozen but who subsequently turned out to have been clearly in violation of reasonable PayPal terms and either hadn't read those terms or had read them but hoped no-one would notice the prohibited behaviour.

I've also heard nightmare stories about many other providers, including about the shiny new startups of recent years. But again, without knowing the context and the overall scale of their businesses, it's impossible to make any objective judgement about how likely it is to happen to you if you're a merchant who is just trying to collect payments and does nothing wrong.