The reasons it is not considered a good idea are that the birth order is important to kids and having their place as the eldest, or eldest girl (or boy) 'usurped' can be difficult. This happens in blended families and various posters have explained how they have done it or are trying to do it, so it does happen but many of the posters are foster carers first and in adoption without fostering first it is very rare, in my experience.
In terms of behaviour, an adopted child will sometimes/often/mostly be developmentally delayed either emotionally or academically, so being the oldest and biggest but needing the most support can be hard. Hard for the adopted child (competing with a younger sibling who naturally has more needs due to being younger).
For the birth child they are competing with an older but possibly/probably needier child. The adopted child may come into the home with bad language, violent behaviour etc. Keeping a younger child safe from a physically bigger, possibly aggressive, certainly needy child is much harder than the reverse.
Can I ask why you want to adopt rather than having another birth child? You don't need to answer.
Just for the record our birth dd is a teenager and our adopted son is 7 and has been with us over 3.5 Years, half go's life.
There are almost six years between them.
This is a bigger gap than I planned but works well.