Have you ever given old company laptops to employees for personal use? Any issues to watch out for?

Check in with your payroll/tax/finance team. Depending on your local laws this can be seen as a taxable benefit. If the devices hold value in your books some accounting practices need to be taken care of too.

Some countries also require you to offer warranty even if given away etc.

Make sure the Data Shredder program is a bootable one so it can delete "everything" do not use programs that run from inside your OS since those tend to leave information in the disk.

Yes we have done that. HD are first scrubbed with a data shredder program to delete all the data and a new OS is installed from scratch.