It is not as easy as taking a liquid product and just removing the water, compress it into pellets and pack into bags or compressed tablets.
Many products actually contain ingredients that are liquid (think about glycerin, or emollient ingredients like short chain glycerides, esters etc…)
If you refer to soap, then you do not have much choice, as the definition of soap is “alkali-metal salt of fatty acids”. Dissolving, say, solid potassium cocoate in water just doesn’t happen without hot water, mechanical stirring, or waiting geological times.
Syndets are usually more readily soluble (aside from a few that require co-surfactants, of which SCI is an example).
Among technological challenges, the first is the protection of your formulation from moisture, moisture is the enemy of powders. If you want to mix in humectants, hydrophilic emollients and so on, those are tendentially highly hygroscopic. You will have to use impermeable bags or sachets. Even those, over time, will let moisture in, the product will cake into a “solid” stone, or into a honey-like saturated solution of your formulation.
The finer your solid, the faster it will dissolve. Thus, microbeads <1mm or finer powders will be preferable to pellets. If opting for microbeads, or tablets then a dispersing agent is needed (modified celluloses, povidones, or disintegrants like citric acid+bicarbonate).
If you use powders, then anti-caking, flow agents are needed (silicates, for example).
I am not a big fan of such products, in general. One issue for which there seems not to be much written about is preservation. Unless the pH of the final product is alkaline or very acidic, the product will not be preserved once diluted with water. You may want to add preservative in the correct target percentage to the powder/bead/tablet, but some may lose efficacy by decomposition, reaction, or evaporation.
Then again, you have the customer side. And customers don’t follow instructions. One of them might decide to use spring water from the well to dilute your powders, that will be far too much of a bacterial load for the preservative to be effective. Another one will use tap water, with high dissolved calcium and magnesium salts, and perhaps some dissolved iron, that will cause (a) precipitation of fatty acid salts (soap scum), and (b) premature rancidity of ingredients (iron ions). Another customer will decide to save money and the environment and dilute your formula twice the required amount (or save half packet for the next time) and your preservative will not be effective.
The bottles that most companies seem to use are made of glass (great for recycling, multi use, yeah!). Which is fine for something that sits on the sink. Definitely not something you want to keep in your shower, where it can slip off your hands/caddy, and shatter into tiny little razor-blade sharps on which you are going to step/fall on.
And this is only for hand-soap / shower.
You cannot make a lotion/cream without some kind of high-shear. Solid products needs to be melted for the emulsifier to work. You need at least 2000 rpm stirrers to produce stable emulsions (with traditional emulsifiers). And if you want to avoid the need for melting stuff, cold emulsifiers require ingredients that are liquid, or readily soluble in either water or oil phase to work properly (and still need some stirring and shaking). Again, the preservation problem will be even more stringent, as in a lotion, with water, you don’t have much leeway in terms of pH adjustment, and you have a lot of germ-food in there. Then there is the fact that you’ll be re-utilizing containers. This is against any good manufacturing practice (where containers are sanitized or sterile, water is purified and/or sterilized, equipment is thoroughly cleaned, and every single documented protocol maintained as clean/traceable as possible).
There are too many things to consider, and 90% of them are things that you can barely control from the customer side. You better sign up for the best liability insurance there is out there. Your formulation is only 10% of the success of your product.
IMHO, a bar of soap, or a syndet bar (that with few modifications can become a solid shampoo), have the same environmental impact as what many startups are trying to do. And I don’t know of anyone who used a bar of soap incorrectly.