Hey lancelot, welcome to the board.
the list is fine to me, its totally playable and should wreck some faces with all that firepower.
However, you asked if that list was good for competitive, whic i assume means tournaments...so my answer is no, here's why:
(mind you, i keep myself a mile away from tourney scene, since i despise that with all my heart, so take my opinion with lot of salt...there are more qualified people that can help you!)
-forgeworld model (not all tourney allow that, check this out first)
-the exterminator loadout is meh...not reliably performing. i'd never mix shortrange melta with long range autocannons.
-extremely low bodycount, squishy models (20 zombies are not so impressive, 10 cultists will die to a handwave, 1 obliterator is negligible, 1 naked sorceror is a token, 5 havocs are just 5 marines) you will remain soon with vehicles alone
-due to above, you have 0 capability to bubblewrap your precious vehicles and protect them from grav-melta-assault. your vehicle gunline is going to have an hard time trying to survive
-you have only 3 vehicles with av 14 (front) with the rest being av12 (imperial artillery) and 13 (grinders on the front) wich means they will not withstand even autocannon fire. due to above, they will be outmanouvred and assaulted/shot to pieces (remember, eldar jetbikes with mass str6 fire is a common thing and 3/4 of your mech will fall to that if shot on the back)
-it will be hard to contest objectives due to lack of speed, manuevrability and obsec units. you are not going to win with points...
-even if you face 0 cc threats, and engage in firing competiton, you cannot prevail against true gunlines (tau): a tau gundam army will just rip you apart in a couple of turns of shooting
-moreover, you will suffer a lot from deployment shenanigans: null deployment will f*k you up; you don't have any trick to mess with opponent deployment (barring that awful unmitigated deepstrike that you could try with the grinders...); you don't have enough bodies to prevent infiltrators to munch you from the sides...also, considering that turn 1 charge is a thing...you cannot protect your vehicles from that, as i said above.
this is my pessimistic idea of how will the list do, competitively.... nothing personal
Suggestions to improve the list:
-drop the forgehost
-drop the cultists
-drop the exterminator
-add 5 IW obliterators
-add 40-60 zombies
-add at least 2 artilleries (hertshaker can do)
this will still leave you in the same spot, but with some extra bubblewrap for your tanks and more firepower.
other nice thing you could do is to add LOTS of MSU screamers instead of the forgehost, or add some summoning battery with tzeentch stuff.
But at this point i hope someone else will have better ideas, i'm not reliable at all, i warn you!
GLORY TO CHAOS!