I think that they've done a pretty good job in retooling Hot Spot as Onslaught. The toy is distinctly Onslaught in both robot and vehicle modes. There's very little that I can fault on this toy. The only minor issues I can see are:
* The "emergency stripes" on the sides of the legs. They've tried to downplay it by replacing the red with purple, but it still seems needlessly stripey. I honestly would've preferred if they'd just made the whole thing silver to mute out the sculpted stripes. They did something similar to this with Scattershot to mute out some of the "Silverboltness" of the mould, and I think it works reasonably well on that toy.
* Similarly with the siren/strobe lights. I would've preferred for them to just leave it uncoloured, that way rendering it the same blue as the standard plastic. I can understand the desire of having purple there to break up the blandness of the blue, but that could also be done by painting in the indented rectangles on the shins (which would also emulate the shin stickers on the G1 toy). These are really very minor issues. On the whole, I think that they've done a bang up job with this toy, and I'm extremely happy with it.

In terms of posability, I find that both my Hot Spot and Onslaught tend to be back heavy (possibly because they're carrying the gestalt robot chest and head on their backs), however, there are heel spurs that you flip out from behind. These work reasonably well in preventing the figures from falling backwards.

---Pics---