How is it rubbish? They felt at home, had the same rules in both houses (the biological parents coordinated on this), got twice as many presents and had a place to go if they wanted to. They gained two half siblings, got to go on two sets of holidays
Key thing here is bio parents coodinated. In many divorces, it isn’t an amicable “grew apart” situation, it’s often affairs, abuse, hurt feelings, often financial spilts aren’t perceived as fair.
Often parenting isn’t on the same page either. One will feel like they do the bulk of the child rearing, which was ok when they were married and a partnership, but doesn’t work as well when that partner isn’t getting the financial benefits of a wage earner in the household, as that wage doesn't support two families so there will likely be a noticeable drop in standard of living.
As pp have said, it’s often how the adults deal with the step situation. The nature of divorce often means resentment and bitterness on some level.