Serials were the "street urchins" of the business. Agents always warned you to never admit that you had appeared in one because it would be held against you be the major studios, although many actors went on the become super stars after getting their start in these films. Some of the top producers, today, got their early training with these serials.
My own regret, now, is that I didn't make more of them. I, too, was reaching for the top rung of the ladder and not wanting to be "type-cast",I turned down three times more serials than I made.