Except for the Tiger, Rootes cars were always a little too heavy for their limited power. Solidly-built is a plus, unless you don't have the ponies to make it go. Plus, like other Brit cars, they were designed first for mostly slower English roads. Imagine using your Sunbeam as a daily driver back when the universal U.S. freeway speed limit was 70mph, with Rootes cars having mostly gearing above 4.0:1 and most lacking the optional OD. A friend whose father owned a Rootes dealership up in B.C. said Sunbeam engines weren't considered very dependable until the Arrow came along with its 5-main bearing engine coupled to a single Stromberg. That one was still pretty slow, but he said it didn't come back to their repair shop as often.
Rootes had other, well-known problems as well, such as the rushed Imp introduction, the Easidrive that wasn't, and the SuperMinx line, which ended up designed too big to replace the current Minx range as intended, leaving an unwieldy number of offerings for a relatively small company.
A 1977 book called Chrysler U.K. - A Corporation in Transition, points to comparatively lackluster profits in the '50s, which probably limited U.S. dealer expansion. It also may have been that Rootes deliberately kept profits low for fear of being nationalized by the '50s Labor govt.
Dick Sanders
Kent, WA