When I had my engineering company we had a couple of parts that needed anno and the company we normally used were mega busy so "we" had the bright idea of buying one of these DIY kits
The only problem we found was...get the right shade of (in our case) red
There's a reason why the professionals are professionals they know what they are doing
If you've got the time (and money) then they are worth a play with but remember different grades of alloy will need different times/rates with the dyes