If there is a texture difference between sanded mud and the drywall, it would show with flat paint as well. Maybe not as much.

Eggshell shows flaws more than flat, definitely.

They need to seal the drywall with specific drywall sealant, THEN start the priming and painting.

Do you think they need to use a different primer if they are using egg shell paint rather than flat?

The higher the sheen of a paint, the better the wall needs to be sealed so as to maintain an even sheen of the paint. Do not use a PVA sealer. Use a 100% acrylic based primer, as it seals better. To absolutely get the best end product, do a primer and two finish coats. If you are going over newly textured walls, this is even more important, as that texturing is nothing more than a slurry of drywall mud and is extremely porous.

If this is new work, you should not have to be worrying about imperfections in the wall - there should be none!

I think eggshell is borderline in its forgiving ability as to sheen. Satin and above definitely requires better primers and/or two finish coats.

Seal it with 2 coats of Gardz and paint away.

Seal it with 2 coats of Gardz and paint away.

Amen! Keep it simple.

Is it more common to use flat or eggshell on walls? Thanks for all the responses.

Eggshell is what I usually see.

It's about 50, 50 for me

I use Behr Ultra Matte in my house, but then, I no longer have kids or pets. For a more active family, in my contracting days, I used to recommend eggshell to customers, at a minimum for high traffic areas such as halls.

I repainted my daughter's interior with Matte and it's holding up with, even with 3 children, 4 to 10 years of age.

I just did the in laws new place with eggshell on the walls and flat on the ceilings. They wanted a scrubbable finish. It is that. Looks decent. I prefer the matte enamel myself but my new house the architect specified eggshell and I didn't change it. I will repaint my interior soon enough anyway.