For me storage space is not the main criterea, it's the transfer speed that is crucial.
Using USB to transfer all my music and photos to the phone memory takes eons. FTP is 10 times faster but you can't beat putting an SD card into a laptop card reader to perform the transfer.
I find my self out of cell phone signal range often so I do like having my music stored on my device. Camping in the mountains, no signal, makes sense. Working out at the gym, lousy signal & wifi. Go figure ...
On my Samsung phone, there is no SD card slot, so obviously, the media is in internal storage. When it's time for a new phone, I use Samsung's Smart Switch to move the files. It's fast & efficient.
On my tablet, my media is still on an SD card. All the data on the card is backed up so if it fails nothing will be lost. If you use a SD cards, you would be wise to back it up. Years ago, I had a corrupted card. What a mess.