Beef – Rib Roast Rare standing up or lying down

Dry and even heat. There is still a difference, even with the same cut of meat and using a roasting rack to avoid roasting the cut in its juices: meat on a rotating skewer will be exposed to uniform heat. If you set the temperature of a home oven to 350°F, the top of the roast will be exposed to that temperature, but the bottom, in line of sight to the heat source, will be much hotter.

Commercial rotisserie ovens also cook the meat by convection and use the heating elements to create the outside crust. As the meat roasts, it becomes darker and absorbs more of the radiant heat from the roasting coils. So the outside ends up getting overcooked unless the roasting coils are carefully controlled, which is kind of hard to do. In a convection oven the temperature is constant around the roast.

I think you've answered your own question. Browning and adding veggies later will both help.

Browning doesn't "seal" the meat to keep flavor or juices in, but it does create a very nice flavor that's almost always associated with meat, caused by the Maillard reaction. I think browning could go a long way to resolving your flavor issue.

This is a little counter intuitive of cooking with a slow cooker, but you can start the meat with a bit of water (or wine, stock, other juices) and then only put the vegetables in much later in the process. Of course, then you have to tend your crock pot, which is often not the point. That would decrease the time that the vegetables had to affect the taste of your meat. You could even experiment with which ones to add later and which to add at the beginning. If it's just the carrots that are offensive, that may be the only thing you need to throw in later.