They're about as good as you could expect from a major cloud storage provider. In theory the encryption means that Mega can't access your files, but they're expressly very cooperative with government agencies so don't bet on anything you put there being entirely secure. I haven't heard of any major problems with them though - it's what I've been using for cloud storage the past few years and I haven't had an issue. As long as files aren't shared and therefor at risk of being reported there's not much to worry about, though in the case of things getting reported it's a 'take down first, ask questions later' type deal.

Here's a transparency report from them (how much to trust it is up to you): https://blog.mega.io/mega-transparency-report-2021/