How do I bathroom / kitchen / extension? etc.

Comes from the same dark place as the hatred of Kitchen Islands a few days back

I think if you have 10k or whatever to get decent aluminium ones with spares easily available they are a fine choice.

They are not uncomplicated things however so I wouldn't want to get a cheap one and then find spares were not available or the mechanism was poor.

I'm getting a sliding door for my extension.

They look great when open but cheap ones in smaller openings look like a sea of messy plastic when closed.
Our doors are prob closed 70% of the time so we went for sliding to have as much glass as possible.

Just good old fashioned snobbery. They're good in the right context, like your place by the sounds of it.

If you have a small deck/terrace then you have to move everything before opening the doors.

They are passé - the market is flooded with cheap versions of what was innovative and expensive 20yr ago.