It's not where wood is joined,it's between panel and frame. It's not a door material problem,it's a finishing problem. Panels should be finished before assembly. If you ask enough people,you will get different opinions,how many designers have you talked to? The designer doesn't live there,you do. Use what please's you.

A painted cabinet is a painted cabinet - it has its own features and properties. Many cabinet doors have a wood frame and an MDF raised center panel. That means the wood in the panel won't expand or contract but the frame is still hardwood. Ask if that's what she's talking about or is it an MDF panel that's been routed out to look like a raised panel door. If it's the second, then you'll have an MDF panel that looks like it's been machined to look like a raised panel door.

Don't be scared of wood doors, they've been making them and painting them for centuries.

My hubby designed and built all of our cabinetry for our kitchen remodel. The cabinet doors are actually solid maple for the shaker style frame part and the inside of the panel is actually maple plywood. The plywood is still a floating panel inside the door part... that gets kind of technical about the building procedure, but it was cheaper than using solid maple that he would have to join... He sprayed the paint on after the doors were fully assembled.
You can see in the closeup photo that there are some hairline cracks in some of the seams where the shaker frames meet. I can honestly say they don't bother me a bit... they're not at every seam, but they are here and there. To me it is just a reminder that we have real wood under that paint and it's part of the character of the kitchen. But you would have to judge that for yourself, what bothers you is likely different than what would bother me. Our kitchen is a mix of solid wood and plywood, but my hubby refused to work with MDF. (The cabinet boxes are also maple plywood, the insides of my cabinets and doors are left a natural color with just a clear shellac coat on the inside.)

Look at both MDF and painted wood doors in person before you decide. My KD recommended the same thing, I put the two doors side by side, and the MDF looked really fake. I am fine living with normal hairline cracks.

we have mdf as the center panel of our painted maple shaker cabinets. we have multipart wood crown molding in adjacent rooms. it took 3 years of repainting the cracks in the crown molding in spring before the wood got 'settled in'. our cabinet doors have never had an issue. i'd listen to your designer.