Bringing back older characters isn't intrinsically wrong IMHO, but only if they are re-introduced to the audience with an assumption of unfamiliarity (ie, don't assume the audience already know them, but still provide fanservice for those who do). A tricky tight-rope to walk. When Sarah came back in the modern series they did this rather well, but it's debatable whether K-9 And Company might have assumed too much.
My problem with K-9 And Company stems from the idea that it was supposed to be a pilot. As a pilot, it gives almost no indication of what a series would be like. If it was going to be 6-12 episodes of Sarah and K-9 investigating oddball and unusual stuff all around England, as a kind of The X-Files before The X-Files, then it would have been much more effective to have just made it a show about U.N.I.T. One thinks they were trying to go for glamour, what with Sarah being an investigative reporter and all, but the so-called 'pilot' doesn't really convey that either. It sends some very mixed messages about what sort of show it would have been.
My problem with K-9 And Company stems from the idea that it was supposed to be a pilot. As a pilot, it gives almost no indication of what a series would be like. If it was going to be 6-12 episodes of Sarah and K-9 investigating oddball and unusual stuff all around England, as a kind of The X-Files before The X-Files, then it would have been much more effective to have just made it a show about U.N.I.T. One thinks they were trying to go for glamour, what with Sarah being an investigative reporter and all, but the so-called 'pilot' doesn't really convey that either. It sends some very mixed messages about what sort of show it would have been.