Robin2:
When I consider the OP's requirement "review similar questions before posting a question/comment" I'm not convinced that any other search system is likely to be better than Google.Unless the request to the search engine uses words that are contained in a Thread that Thread won't be found. For all its qualities, and the huge ongoing investment in it, Google is still really primitive compare to the human mind.
Well, it's certainly an advantage if system actually indexes every forum page. Google does not. An on-site search will have every single page from the moment it hits the database.
It's very common that people are posting here about an error message. Searching for that error message is very likely to find the previous posts (as long as they didn't use a screenshot of the error instead of text). The key is to have good search skills.
If you search for this:
"C:\Users\per\Documents\Arduino\libraries\max6675arduinolibrary/MAX6675.h:9:22: error: WProgram.h: No such file or directory"
You're not going to get any results.
If you search for this:
"MAX6675.h:9:22: error: WProgram.h: No such file or directory"
You might get some results with very specific directions for fixing the problem in that library, or you might get nothing.
If you search for this:
"error: WProgram.h: No such file or directory"
You're going to get plenty of results that are reasonably relevant.
If you search for this:
"No such file or directory"
You're going to get thousands of results but most will only give a general hint about the cause of the problem.
If you search for this:
"exit status 1"
You're going to get a zillion unrelated results.
It's all about being able to craft a search that is not too specific and not too general. That's a basic skill anyone using the Internet should have but I'm not convinced they do.
In other cases, there might not be a specific phrase you can search for. In that case, the "fuzziness" of Google's search may be an advantage for this purpose and it's unlikely that an on-site search feature would be able to do that sort of thing anywhere near as well as Google, if at all.
Robin2:
I don't have confidence that the Arduino developers could do a better job - particularly as they don't use the Forum regularly and cannot be aware of their customers' experience.
It's not a matter of them writing a search engine from scratch. They should be able to just grab an existing solution and install it. The search functionality is already available for the SMF forum software. The use of an outdated and modified version of the forum software might make it not so simple though.