As the title says and hopefully self explanatory to most.
They are extremely good at acting as meat shields to run (or TP for Eldar) into key ranged units. making units like devastators and wraithguard way harder to use in practice
You should have to commit actual units that cost resources to tie up dangerous ranged units - you know, cost vs reward.
I believe the game will feel much more fair and intuitive if workers couldn't melee.
Not as critically, I think builders shouldn't be allowed to capture points - one reason being that it complements rush strategies too well by having builders capture points to build gens on - saving precious time to start sending your rush/spam to the enemy (in dow1/2, this is counteracted by sacrificing map control since they could not pressure and cap at the same time).
Builders only role should be build, repair, detect/recon.