I'm a long-time user of Spectacle [1], but it's no longer maintained. Glad to see what looks to be a good alternative.
If you haven't switched, Rectangle comes with a Spectacle compatibility mode so all the keyboard shortcuts and behaviors transfer. When I swapped out work machines a couple years ago I cut over and it was painless, fwiw.
TL;DR The APIs are the same between the two, but there is the potential of a security risk with Spectacle. Something I didn't mention in that post is that Rectangle technically should outperform Spectacle, although it might not be noticeable, as Spectacle loads a JavaScript engine to perform window size/pos calculations while everything in Rectangle is in Swift.
[1]: https://medium.com/ryan-hanson/why-switch-from-spectacle-to-...