To recap ...
The Scarab SRV is a difficult vehicle to control and takes skill and practice to master. General consensus is that it behaves better with Drive Assist turned off although personal experience can vary, especially if you're using digital throttle inputs and like the idea of setting the SRV's forward speed rather than having direct input to the wheel speed. The vehicle's speed (which is displayed in m/s) can be misleading (i.e. 30m/s is actually over 100km/h) so it's not entirely surprising that it's difficult to control at full throttle, especially since we're typically driving on low-g worlds with icy surfaces. One simple solution to control problems is to just slow down until your skills have improved.
There's currently a bug applicable to driving with Drive Assist turned on where mousewheel and/or digital input(?) can cause the SRV's speed to suddenly leap uncontrollably to maximum. You can find further forum threads about that
here,
here and
here and also a bug tracker issue that you can contribute to
here.
There's a certain amount of resentment towards the SRV racing community who campaigned quite hard to keep the Scarab's handling as it had been in Horizons after pre-launch testing in the Odyssey beta revealed it had been messed with considerably. There's no evidence to suggest that Frontier ever intended for it to release in that state but regardless, when it finally launched handling had been "largely" restored (although it still has an unnecsssary self righting mechanism which means, for example, that it's now difficult if not impossible to pop the SRV onto its rear in the
"praying" position). There's also the possibility that the handling changes we discovered prematurely pre-launch (via the challenge scenario) had been left in as a way for FD to try out characteristics we'd eventually see in the Scorpion. Some commanders still feel that the Scarab should be thought of primarily as a ground hugging ATV but, whether by accident or design, it's proven to excel at bounding across the landscape using its boosters in a style known colloquially as
"flyving" and I personally think it's unwise to try and denounce that.
Many players actively engaged in these discussions feel that the current Drive Assist option isn't that useful and that it would be better repurposed so that Drive Assist off leaves the SRV with the Horizons handling model while turning Drive Assist on employs the RL equivalent of things like active differentials, ABS and a more active rear wheel steering mechanism to give the Scarab a more refined and less sporty performance model.
Loading up the SRV with cargo does not affect its weight or handling, nor does having the cargo scoop deployed. The SRV's shields do not protect the vehicle against ground collisions and, unless engaged in combat scenarios, it's therefore pointless putting any pips to SYS and drivers are advised to keep 4 pips in ENG at all times (unless actively attempting to restrict the vehicle's performance, either in an attempt to keep the beast under control or to abide by the regulations of specifc, more ground-born, race variants).