@draconiam well if you are talking upgrades, then why not just upgrade the better vehicle. Take the Mako, add longer boost for greater speed, improve suspension to be more like Nomad of ME:Andromeda, improve the shields and armor more so it is even tougher, add a missile system to increase firepower and then you have a absolute beast of a vehicle.
While the HH can be improved, it generally starts worse than the Mako, thus it is a worse vehicle. When you can RAM a Geth tank, knock it down, boost high up and land on it's back and kill it faster than shooting it, is an issue. When you can RAM a heavy Geth tank and take no damage, yet a standard Geth trooper with a rifle can kill you in a few bursts, there is an issue. When the tracking missiles can't seem to lock on to a target and hit it, so you have to spray and pray is an issue.
Yeah, Mako for me, as if you pay attention to the terrain you can follow the "paths" Bioware put in to almost all points of the map, thus not having to "crawl straight up the mountains".