Hi
Ok, I don't see why an 'in-frame' kitchen can only be made from solid wood... and there are examples of people making in-frame kitchens from MDF or vinyl wrap.
Knots may bleed through top coats, or they may not... no material is perfect in all respects.. this could be considered a downside, but it is a minor one in my opinion. High quality pine only has dead knots, not live seeping knots and so I reckon a specialist product, like a knotting fluid, or a primer like Zinnser BIN is sufficient. Customers want to strike a balance between cost and suitability...
Poplar is also a (physically) soft wood and can be dented easily enough. I have used it on occasion.. i believe it is often used for painted work as it is relatively cheap, straight grained, relatively knot free, and it has a greenish tinge which renders it unsuitable for many other purposes. Also, being a solid wood, it suffers from warping, and expansion and contraction which can cause unpainted areas of central panels to become exposed. Using poplar will increase the cost when compared to solid pine, and so I would recommend the use of pine rather than poplar to cost consious clients.
What other solid woods can be used in carcases, and in what respects are those woods more suitable than pine? I can't imagine them being more suitable in terms of cost.... and I can't imagine them being more cost effective as there can only be some small benefits when compared to pine and quite possibly a large cost difference.
PU is better than AC, but not by miles, and it has its disadvantages, like pot life. AC had been the industry standard for years before the introduction of PU. PU is very popular in countries like Italy.. not so much here although it may become more popular over time.
Originally I only posted a very short post that directly answered the OPs question... then I responded to what I felt were inaccurate comments.
I fully accept that different materials have different pros and cons and that no material is super fantastic and perfect in all respects... so that is why I let my customers choose for themselves... many of my customers are my customers simply because they do not want MDF, not at any cost... and so I will make them what they want... mostly they don't want MDF for enviromental reasons, or health reasons (Formaldhyde etc), or reasons of longevity.
Cheers so,
Joe