Apart from any temperament problems with crosses you also have the potential for double the health problems. I know some people believe in hybrid vigour but I am afraid it is an urban myth, but you might just get off with it in complete and totally mixed breed mongrels.
For instance, if you have a Labrador with hip dysplasia mated to a collie with Collie Eye Anomaly/Choroidal Hypoplasia (CEA/CH) or Ceroid Lipofuscinosis (CL) then you could be breeding pups with any or all of these conditions. A pure bred can be checked for the recognised health problems - for instance in my breed hip dysplasia has been known so we x-ray breeding stock. I only breed when the hip score is below the current breed average - at present it is 17, my dogs are 7, 7, and 14.
If you have poultry you must remember most gundogs are bred to find and retrieve birds(some are bred to kill - GWP are used to find and kill feral cats in Australia), so an adult rescue could come with a built in wish to retrieve your stock to hand. I have to keep my birds penned up when the dogs are out, they have been known to bring my ducks and chickens back to me - alive and kicking as they all have soft mouths - except my GWP chicken killer !!
). Terriers are bred to kill, hounds are escape artists and take off if not on lead or securely contained. The herding breeds are good for farms provided they are trained to herd and not chase. A Bernese Mountain dog is unlikely to stray far but they get very warm in our climate, as do Newoundlands and other similar heavily coated breeds.
I can recommend gundogs as a good useful all round family dog, but there are so many different breeds that it is impossible to generalise. Why not try the Kennel Club puppy finder.
http://www.the-kennel-club.org.uk/findabreed/; then check out the health recommendations for the breeds that appeal to you. You could, of course, go to the local cat and dog home and take home a mongrel that needs a home and that but for you would be put down. It's your choice.