They have over a decade of experience in sandbox games, very true, but that doesn't mean whatever fundamentals they have set in their recent games cannot be surpassed. Quite the opposite, post-morrowind Bethesda has been on a slow decline, example being how a lot of the original designers left the company due to internal politics. Presently there's really two things they're good at: marketing and world environments.
Their game design is incredibly weak (compare Van Buren's plot to Fallout 3's search for NPC_Liam_Neeson, purify water plot and don't get me started on Oblivion.) Q&A? I have the notion that their "bug testing" team consist of offshore outsources from a sweatshop in China. It says a lot about how much a company cares about the quality of their products when the people who pay money to support their company have to literally pay to fix their games for them.
They are very aware of this, hence why they want to have tight control over any piece of news regarding their games (ever notice how so little news is released up until a few months before release yet they are their own publisher that doesn't have to bow to any external companies? Not including Zenimax.) But during those few months, anything that is critical of their games is quickly dealt with. All those "review" articles that give their games 9, 10, GOTY A++? Subtract 25-30 points and that's the actual score if they were not bribed.
Picture Oblivion in all it's ****iness and fail for a second, now imagine it having deep quests with multiple solutions, fleshed out setting with areas and factions that actually have a backstory to them, characters that were well written and NPC's that didn't use the same four voice actors. Could Obsidian do that? If they had focus on what they wanted to accomplish in such a game, I honestly think so. The only real flaw that I see in Obsidian's products is a lack of polish, that's a lot higher than post-morrowind Bethesda.
Gamebryo is a good, powerful engine. The problem is Bethsoft's software engineers and the fact they primarily develop on console platforms. If they developed primarily on PC then ported to consoles, the overall performance would most likely be better on all platforms. Ironic how they focus a lot on graphics yet static objects always have their shadows disabled (requiring tweaking in .ini files to enable.)
/rant