It's not an unreasonable opinion. Google is dependent on a single product, there's almost no barrier for users or advertisers to switch, and most of Google's proprietary technology in software and datacenter design is completely opaque to investors.
I'm not sure Google is a slam-dunk investment even today. They have their fingers in a lot of pies, but the company is still entirely dependent on an advertising product that hasn't changed substantially in years. If the market changed in a way that disrupts adwords, things could go south quickly. They remind me a bit of Microsoft - very oriented on protecting a lucrative monopoly, but in doing so their other products tend to be designed to support the monopoly, not generate revenue on their own.
If you look at what they actually have in AdWords, it's not easy to see how it could be seriously disrupted. You're right that Google's real monopoly is not on search but on their immense network of advertisers and content sites, which has economies of scale built in that benefit everyone involved. Simply by being the biggest, Google's network is the best. The only thing that has come close to challenging that is the walled garden of Facebook, because of its unprecedented scale. Now Facebook surely has some growing left to do but even they would still need to pretty much cannibalise over half of the web's pageviews to have a shot at usurping Google's ad network.
Compare Microsoft, whose monopoly positions are relatively brittle. As we see with IE9, if Microsoft slips to even 66% market share (that's the current US market share for Google Search[1]), they need to start thinking seriously about interoperability. An advertising monopoly is something much more fluid. I think Google would have to majorly drop the ball to get toppled in the medium term (maybe failing to respond to an extreme price challenge over several years from a large rival network, or some really catastrophic privacy blunder).
These days they have a good hold on their position. But back then, if IE had shipped with ad-blockers built in, Google would have had a serious struggle.
I'm not sure Google is a slam-dunk investment even today. They have their fingers in a lot of pies, but the company is still entirely dependent on an advertising product that hasn't changed substantially in years. If the market changed in a way that disrupts adwords, things could go south quickly. They remind me a bit of Microsoft - very oriented on protecting a lucrative monopoly, but in doing so their other products tend to be designed to support the monopoly, not generate revenue on their own.