Especially older versus newer adapters. The old heavy ones weren't usually regulated, so e.g. a 12V adapter would put out 17V open-circuit. Also a lot of them used negative-tip polarity instead of the near standard positive-tip now used. Then the older device would use an inefficient linear regulator. Some devices like routers (that I've seen) still use an internal (switching) regulator so you can feed them with more than 12V, but others expect regulated voltage. Newer lightweight ones are almost all positive-tip and regulated. Cheapo devices have cheapo adapters which I wouldn't put on an expensive device. Also they tend to cut power when the current rating is exceeded, so for something needing a 2A 5V supply don't use your cheap 0.5A or 1A phone charger unless you want random device reset when it happens to actually need 2A.