When travelling by road in a personal vehicle with your pet it is only as risky as it is for you. By train, it may be a bit more difficult as the pet has to travel as cargo in a kennel in train but still, there is a lesser risk as compared to air travel like temperature or pressure drop etc. The air travel can be a little risky because of factors extreme temperature and pressure changes. Even though airlines maintain temperature and pressure for pets in cargo hold area also but what if things go wrong.

Breeds with a snub nose like a french bulldog, pug, boxer, Pekinese and Persian cats  etc are usually not allowed in the cargo/hold area as they have been found to develop breathing problem to their nose. They may travel in the cabin.

At least 10 weeks old.

75kgs combined pet + kennel. This may vary a bit with airlines but usually, this is pretty much a standard. Anything more than this you will have to use cargo services.

2 adult animals of comparable size that are compatible, up to 14 kg (30 lbs) each, can travel together in 1 kennel. This also goes for 3 animals up to 6 months old from the same litter, up to 14 kg (30 lbs) each.