The usefulness of goliath is more or less irrelevant in 1v1. Games rarely get to the stage where goliaths can be fielded in any signficant numbers. Still, it's a cool unit, so it's a pity that it usually isn't worth the cost. I generally only make it when I can morph it from a reaper and thus enjoy the benefits of free buildpower and reduced rally time (can morph it near the battlefield).
Goliath mainly suffers from two problems:
1) Role overlaps too much with reaper
2) Gets outclassed by other units of it's price range in terms of what it brings to the table
What's the point of building a goliath when you can just keep making reapers? Sure, higher weight allows it to survive even better than reapers, but it's also slower and worse vs most (almost all) units in a straight up fight. A goliath isn't usually scarier to face than the same cost in reapers.
Here's the list of other units in goliath's (2200 metal) cost range:
Grizzly - 2000 metal
Sumo - 2000 metal
Ultimatum - 2000 metal
Crabe - 1600 metal
All of those units share a trait - they all excel at something. Something that other units of these respective factories don't offer. Be it the long range skirm/assault grizzly, be it the ultimate antiheavy ultimatum, be it the crabe that can single handedly take down whole armies with it's AOE or be it the generalist sumo, which excels at crushing blobs of anything while being able to duke it out with heavies in a close quarter fight.
I'm going to pick the sumo and compare it to the goliath a bit more in-depth.
Sumo is by design quite a general-purpose unit. It has an AOE crush ability, which allows it to be devastating to cloaky and shieldballs. It also has accurate weapons, which allow it to eat through small targets like gunships, raiders, etc. Because it uses disruptor beams, it is also an amazing single target disabler, being able to duke it out with other heavies in close quarter combat.
Goliath immediately strikes as being much less generalist. Inaccurate main cannon makes it waste a lot of DPS against faster/smaller targets. High alpha makes it waste even more against those targets. It has less HP than sumo and costs more. It has somewhat more range than the sumo, but nothing special. So, the logical assumption is, that due to being much less generalist, it would outclass the sumo significantly in some other area. Sadly, that isn't really true. Sumo beats goliath in a straight up fight(with up to around 35% HP remaining depending on how they engage), which is one of the few things goliath is supposed to be good at.
The reason why sumo so easily beats goliath in a close quarter fight is obvious. Goliath's slowbeam is of no match to the disabling power of sumo's disruptors. Sumo stays at around 0-7% slow the whole fight, outregenerating goliath's slow weapon(!), while goliath gets slowed down to 50%. The slowbeam in it's current state is only useful as an accuracy enhancer for the main cannon vs lighter and medium targets. It simply lacks the DPS to slow heavies fast enough to make a difference in a fight. Comparison: moderator's (unit costs 220) slowbeam has a DPS of 350, goliath's slowbeam has a DPS of 200.
Unless it's Red Comet, I'd take sumo over goliath any time. Again, unless it's Red Comet, I'd take grizzly over goliath almost every time. I'd even without hesitation take crabe over goliath in quite a lot of situations, even though it costs 600 less. Note that as an additional handicap, goliath is also the most terrain-restricted of it's cost range.
Proposed solution: increase goliath's slowbeam's damage. This would emphasize it's main difference with the reaper. It would emphasize the unit's (supposed) strengths and not make it more generalist. If the numbers are tweaked
right, it would make the unit finally worth it's hefty pricetag.
TL;DR Goliath could use a buff, preferrably to it's slowbeam