Dreads are very powerful. You just need to know some rules.
1) Melee dread/Venerable dread - the main purpose is melee superiority. They are natural answer to super heavy infantry like terminators or other melee units like Nobs/Ogryns. But you need to know that they need support. 1v1 is more comfortable, because there are no unexpected tanks/setup AV from 2 other players from the both flanks like it's often happens in 3v3. So you tie up units and other army support your dread, controlling sources of snare/AV. (like asm for setup and your lascannons vs Tanks). But you need to know Dread can't solo AT or Nobz squad, but can deal significant damage to them. Khorne dread get significant damage resistance on his ability but lose control for this period.
Some tips:
Venerable dread's charge is your dread's life saving ability, you can dodge almost all threats with it, forcing your VD to escape. But it could be used offencively, if you're not facing AV thtreats because of it's speed and 50 splash damage. I love to charge retreating high model-count squads. First - u get damage of charge (50 per model), and reach them to get one more melee splash attack (100 dps). Will be deadly enough.
Emperor's fist is my favorite vs Termies to target Orbital bombardment: you force melee termies, target orbital, and use Fist. Then just move dread a bit so you can avoid rays but still can attack helpless termies. It won't work with nobz because of no knockback immunity - they fly away
2) Melta dread
i prefer it as a counter to t2 vechicles or if i need ranged attack. it's better than AC dread because of more stable attack speed. Dps is quite even.
If you reach tank with melta, you can force melee it and tank will be dead. but it's pointless to attack tank if it's already kiting your dread. You need snare in this case.
3) AC dread - awesome thing vs high model-count squads in terms of it's main ability (Assault barrage), and awful in other way. Because of it's burst nature - you get AC burst in ONE model so if it's low HP one - bunch of damage will be wasted. better attackin the heroes.
But assault barrage is devastating - it can wipe whole GM/gaunt/spider squad, but don't rely on it, it's hard to master. And 2x assault barrage from 2 AC dreads is FUN
ranged dreads lose it's melee resistanse and splash, but melee skill stays, so they still could deliver a good melee special to termies. just count it as lower DPS without splash. Still useful. Khorne dread will obtain melee resistance.
nasty thing - your Melta dread could be tied in melee and you can lose track of your target, which will be alive - so careful!
4) Tzeench dread (chaos) could kill any ranged non-walker, non-super vechicle, but be careful with opponent's melee dreads - avoid melee fights (see tie up in melee)
5) Plasma dread (OM) is more reliable than PDevs so very useful.
6) OM dread (any variant) could get a infantry-stunning ability via upgrade. Very useful, because SM dreads lose EF ability after upgrading.
There is often occuring bugs with walkers 1) you lose control of it. Sometimes it could be returned. But 98% chance dread will be dead to this time.
2) dread attacking transport don't damage it. Because it targets unit INSIDE transport (if it was embarking they are OVER tranport for a short period of time and you can accidentally click on it instead of transport). You better stop it and re-target the transport.
Also melee kills with dreads inspiring nearby troops (Libby also got this ability) so you better fight with your army.
Fun fact: you can lose your dread vs T1 techmarine: if you got snared with his landmine while attacking 2x Vengeance Devastators even if your dread isn't melee but melta one. You can reach one dev squad and even kill it, but Vengeance rounds are so good - your dread will be dead
PS: Impregnable, it sometimes looks like you're forcing people NOT to ask questions on forums. Which is bad. Discord is nice thing, but don't force it too much. Like i see your templated answer in every freaking topic
)