Using Compound adhesive - Tile Advice

[Jrterror]

Hi. I've made enquiries about a shower tray that we are considering buying. More specifically, I asked if this type of tray can be raised on timber etc and then the tray bedded upon a layer of mortar. The reply from the supplier was "There shouldn’t be any problem raising the tray as suggested, although the tray should be adhered using a compound adhesive as concrete can cause the tray to bubble/crack."

I have no problem with the direction to use compound adhesive instead of concrete, however they sent a link for the product and it was a 310ml tune of adhesive (apparently specific for shower trays, haha). My understanding of bedding on concrete is that the semi liquid state of concrete, prior to full cure is that it will fill any irregularities in the tray base, therefore fully supporting the tray across its entire area. Is this correct?
If my assupassum is correct, I would need a lot of compound adhesive to fulfill the same job.
Is compound adhesive used for a different reason, rather than support?
Ta

 

[Simon Dale]

My plumber uses rapid set flexible adhesive. A lot of plumbers even use sand and cement on chipboard flooring. WRONG lol

 

[Qwerty]

I use rapid set tile adhesive on screeds and grab adhesive on wooden surfaces

 

[Jrterror]

Hi. Thanks for taking the time to reply.
Does anyone actually know WHY compound adhesive is used though. Rather than a sand cement mortar.
And why is mortar WRONG on chipboard? Is it also wrong on ply? Because I've read manufacturers instructions that states 5mm of sand and cement mortar on 18mm Ply.