Substituting a sweetener into an Indian sweet rarely produces the same sweet with less sugar. Sugar is doing several structural jobs, and taking it out removes all of them at once.
Sugar builds the structure in crystallized sweets
A cut sweet such as barfi holds its shape because sugar comes out of solution as a rigid crystal network as the mixture cools.
Most sugar substitutes do not crystallize the same way at the same concentrations, so the mixture stays soft and refuses to firm into something that can be cut.
Recipes work around this by adding milk powder, nut flour or set fat to supply rigidity, which is why sugar-free versions are usually denser and shorter in texture.
Sugar controls how much water stays available
Dissolved sugar binds water, which keeps a sweet moist and simultaneously makes that water less available to microbes.
Remove it and the same recipe both dries out faster and spoils faster, which is why reduced-sugar versions are often sold chilled where the originals are not.
That shortened window is the practical consequence most home cooks meet first, usually a day or two after making a batch that seemed fine on the day.
Syrup-soaked sweets have no equivalent at all
A sweet that absorbs syrup depends on a concentrated sugar solution for the absorption itself. There is no substitute that reproduces the density and behavior of that liquid.
Versions made with alternative sweeteners tend to be simmered in a thinner liquid and then sweetened separately, which produces something related but structurally different.
The sweeteners behave differently from each other
Sugar alcohols hold moisture reasonably well but can produce a cooling sensation on the tongue that reads oddly against warm spices.
High-intensity sweeteners provide taste and nothing else, so every gram of removed sugar has to be replaced by bulk from another ingredient.
Jaggery and date-based sweeteners are still sugars, so they change flavor and mineral content without changing the structural picture in any meaningful way.
The honest version is a redesigned sweet
The results that work are recipes designed around the substitute from the start, usually nut and dairy-based, rather than conversions of existing syrup or crystallization recipes.
Anyone modifying sweets for a specific medical reason should treat that as a question for a qualified professional rather than a matter of ingredient swapping.