>>It's not the vertical load capacity I'd be concerned about - it's the ability to resist cyclical transverse (sideways) loads

Yea, that's my point exactly. You've got ~400lb raised up on 4 thin (1/4" to 3/8" thick? x 1-1/2"?) bars (6 to 8" high?). Subject to 1/2 to 1 G cornering loads (??) under normal circumstances, more during peak loads (pot holes, hard swerves, accidents).
From the looks of it, the only thing holding that bar vertical is a small flat plate at the bottom held on by a U-Bolt. The U-bolt is subject to increased tension due to side forces (the support tying to fall over). The weld joint is subject to fluctuating shear and bending loads and the bar is subject to bending.

If you grab the boat while on the trailer, do those supports bend or wobble at all?

I think a fairly simple fix or upgrade would be to take some more of that bar, and weld it at a 45deg angle to the existing vertical just below your hull cradle. Weld a base plate at the bottom with holes for a U-bolt. This would virtually eliminate any bending loads and give the bracket a lot more horizontal stability.

sm