Using a UUID as your primary key can make certain optimisations much harder down the road, most notably sharding which tends to balance really nicely if you do

Using a UUID as your primary key can make certain optimisations much harder down the road, most notably sharding which tends to balance really nicely if you do it on an auto-incrementing numeric ID, and less so if you're using random strings because you'll end up with with some of them clustering. They're also much harder to pass around in cases where you need to refer to a specific record, for example if you've got someone in tech support who needs help with a particular object its much easier to say "its record 18434" than it is to try and read out a UUID over the phone.