It appears as if he either doesn't want to build the built-in or he doesn't know how to do it. We have had two, in two different showers, for years - no slime - no problem.

Good luck!

I've had both, personally a tiled shower bench is too cold on the backside and I would always prefer a larger shower. If you have the space go ahead and build a properly waterproofed tiled bench. The slimy part is bull$&@"

We have a bench made of quartz slab. It's not slimy, ever. We have a steam shower so a bench was a must!

Kerdi system...costly and worth every penny.

Eek warning sign right there! If he doesn't know how to do a bench properly, what else doesn't he know??? Find a new builder!

have you seen his previous work, particularly a seat in a shower? it shouldn't be slimy, and should not allow water to accumulate or stand.
something to note when you get the seat built in; seats that are built down to the floor at the end, rather than ledges, allow much easier cleaning, as you don't need to get to an always damp corner beneath the ledge

What's his problem? Kerdi, tiled front sides etc, and top with same solid surface as the curb. With a slight bit of pitch.. I like it edge to edge, and the glass to the shower right over the bench and fitted to curb, if two walls of glass.

We do all sorts of Benches and niches in our showers not sure why you couldn't in yours?

If you go with this builder, make sure you get in there with a level to make sure he has the correct slant on it. The goober that did our shower slanted it the wrong direction! We were building long distance so we didn't catch that. Neither did our builder evidently. The tile guy got in an argument with his neighbor and is "no longer with us". Some real characters in that area of the mountains.

Even slanted slightly in the wrong direction, it does not get slimy. It's called "housekeeping" and regular use of a squeegy on the tile, regardless.

If he's using wood to frame out benches in showers he's doing it wrong!
Tell him to tile the whole shower and then use bricks and cinder blocks to frame the bench seat. Go over it with a cement board, tile it and you're set.