Some cameras can take smaller RAW-files, which in my case, are about 4K and HD resolution. Are there any drawbacks of using this apart from the obvious reduction of resolution and cropping potential?

The main reason is to save space and time consuming backups.

Anyone done this and regretted?

Hi Steinardo, I'd not recommend using those smaller Raw formats because they usually do not only secrifice resolution but also bit depth and therefore editablilty.
If you only are doing test shots, that are not important for you, sure, go for it.
But for important shots, I'd always go for the full Raw quality.
It doesn't make sense to have a great camera and then artificially sacrifice quality when shooting.