I know nothing about slats having never used them but I do think you can provide high welfare inside
I think you can too, always nice to watch a well bedded, content group of cattle in a shed on a cold wet day.
I've seen some of these slatted sheds too though. No bedding, bugger all space, cold and damp, cattle seem to spend more time standing in the ones I've seen too. Even worse when the grass is growing and the suns shining and they're still stuck in there.
High welfare my arse.