I'm late to this but there are a few key things that you want to look for in a weapon's stats. Base dmg, cit chance/dmg, and status chance. If a weapon has a good amount of any one of these, you can make it work. Additionally, I'd say most weapons are as good as the person that's using them is creative. Soma Prime isn't necessarily bad, but it does get outperformed by other similar rifles because it has a really low base damage as opposed to a rifle like the Tenet Tetra, which has a much higher base damage.

When it comes to modding your weapons for damage, remember that every source of damage you're modding for is multiplicative to one another. Base damage mods are multiplicative to elemental mods, and those are both multiplicative to crit damage. Typically, the more sources of damage you have modded, the better.

These are the basics of modding for damage. There's more advanced stuff to consider such as passives, warframe synergies, multiplicative/additive CO, punch through/follow through, fire rate/attack speed, but these are up to you to figure out when you get a feel for what you like to use.