I tried a different notebook that hasn't had any new documents added in about 2 years. Same kind of search on a unique last name. 104 found on v10 vs 112 found in Legacy. Not terrible, but I would hope that search would be consistent between the two versions.
I did check my ScanSnap options and for the Scan to Evernote profile, "Convert to Searchable PDF" was not selected. I don't know if this was required or not for Evernote to effectively search PDFs. If it is required, it could be that the last update to ScanSnap may have changed that option. It has been quite some time since this software has had an update though, so I couldn't tell you.