My oven is very well insulated, has some very powerful additional elements and has a fan to move the hot air around.
I have no problems in maintaining a very good profile everywhere in the oven. The only exception is in the cool down part of the cycle.
Another colleague of mine has a similar controller used for "public electronics" (we have up to 40 people populating surface mount boards one night a month). His oven doesn't follow the profile so well (it is under powered) but there is rarely a problem. If there is, it is far more common for it to be operator error than a problem in the reflow process.