Covering popcorn in the vaulted main room. Just under 1200 sqft. Eight sections 8ft wide with decorative beams separating them so laying will be simple, just stacking them horizontally up the ceiling.  Wife wants it white. I can get MDF prepainted which would save me a ton of time. Any reason not to go with MDF?

MDF is the way to go.  Doesn't shrink and expand as bad with temp and humidititties.

Sorry, but shiplap is waaaaay too close to SIIHPAPP.

Just my opinion.

Shiplap in a coffered ceiling?

I would maybe mock that up or do a scale drawing or something to make sure you want that look. It sounds... odd. I might be envisioning it wrong. That said, if you go that route, I would not go MDF. Presuming you are referring to 4x8 sheets, it will have to be screwed up. Traditional ship lap can be toe nailed with finish nails in the tongue.

MDF is great on walls because you can use construction adhesive and a few finish nails since you aren't fighting gravity. Not so much when it comes to ceilings.

Never done it, but..

I'd be worried that mdf would sag over time unless glued, adding to the work.

The pine could be finished in a faux pickled oak type finish which would add a little style over plain
white paint. Translucent, but deep enough that you just barely see the woodgain, then covered with
satin poly.

moisture may be a problem with MDF. but it would probably work.

Think A-Frame house style vaulted ceiling, decorative beams running vertically from the 8ft walls up to the 19ft peak. Shiplap will be layed horizontally up the peak. Each run is 8ft wide and I think about 16ft long.

The MDF Shiplap is 5/8” thick, 5.5” by 8’ boards like this:
Attached File

I don't see how you can install that without face nailing in addition to nailing in the tongue as I simply wouldn't trust adhesive on a ceiling install to replace nails. This brings up the problem when face nailing MDF - it displaces & raises the material around the nail head and when you sand it flat that spot is rougher than the surrounding material. If you spray the finish coat on MDF it really shows these spots.

Like others suggested I'd go with a true tongue & groove product instead of shiplap.

MDF is garbage. if it ever gets wet (or even condensing humidity) it’s done.  It swells and never unswellsnaffer drying.  And it’s both heavy and weak… it will tend to sag under its own weight without a bunch of fasteners or using adhesive..

I’d go vinyl planks before MDF any day.  

Wood is best though.

That product does change the situation some. As other have pointed out, moisture is a consideration so I would definitely recommend thoroughly sealing any cut ends. Thinned out wood glue works well. I would probably personally still go with solid wood over MDF in that scenario but that particular product makes a MDF more feasible. Nailing will be a treat. While you can probably get away with a 2 or 2 1/4" finish nail in both the face and tongue, the pressure needed to shoot the face + sheetrock +popcorn +strapping or joist is not the same as the thinner tongue. You're going to want to set your depth and pressure to account for both, lest you find yourself shooting through or trying to set nails on a ceiling with a hammer and nail set.

What are your feelings on VOC's?
How about fire resistance?

Ship lap is out, and siding inside your house is a terrible idea. Ive been replacing most of the "shiplap" I did a few years ago. Even the barn wood ceilings are out. The stuff rains dusty dirt on new furniture.  MDF is garbage and full of formaldehyde.
You'll wonder why you get head aches after you sit in there a while. For the money, get a good dry wall guy and they can sand and skim it. with paint and a good prep it'll take four days. It's a good classic look and way less hassle.

Tons of reasons to not go with MDF.  Just don't.