Braze On is Better
I have never liked these bolt-ons but as I have got older, don't like downtube shifters either.
Most of my restorations for old frames involves respraying and new decals, so I normally remove any evidence of downshifters, and braze a single or double underneath.
If if want to use modern gear then I have to use modern shifters, but I have also installed handlebar top thumb
shifters such as Falcon which are easy to use but of course no indexing. Depends on the bike and who it's for..