Imagine if you will that you are going to paint your house, and when you go to the paint store none of the cans are labeled. When you take the can you bought home you'll find that you may not have gotten the result which you had hoped for when your made your purchase.
When taking a "large piece of venison" and roasting it you are taking the same kind of risk. All animals including deer are comprised of a vast number of muscles, some of which can be tough when cooked and others which may be very tender. Add to this that cooking methods, length of cooking, internal temperature it is cooked to, seasonings, and marinades can all affect the results and you have several areas that need to be addressed when preparing meat.
A simplified rule of thumb is that the muscles used for movement are going to be tougher than muscles that are just there. In most cases (not all) the muscles from frontquarters are going to be tougher than the muscles from the hindquarters. The muscles along the backbone (backstraps) in the center of the animal will be the most tender as they are not used for locomotion.
Meat cutters are skilled in separating and cutting the various muscles into the best options for preparation. If you refer to the "Processing Options" page on our website you will see a picture of the muscles used for steaks and/or roasts off of a deer. The cuts #8 through #11 are from the neck and front shoulders and are best made with a moist heat such as a crock pot, pressure cooker, or roaster.
The cuts #1 through #7 could be dry roasted as long as they are not over-cooked. Ruth and I use pop-up timers set for 145 degrees.
You will note that none of these pieces are overly large, that's because the muscles are connected by tissue that includes silver skin, fat, and membranes. Roasting a "whole quarter" or large piece will usually contain a significant amount of undesirable meat that will give you a resulting mix of both tender and tough pieces.
If you need a large quantity for roasting either use the backstraps whole (or cut in 1/2 and tied together), or use the whole rounds with the heel piece removed.
For overall tenderness in a roast I still believe a crock pot or roaster (with water) is the absolute best option.
Good luck,
Dave