Additional cardholder pitfalls...

You have already identified the obvious pitfalls and other posters have related their bad experiences of adding a cardholder. Only you know whether it is safe to trust the person you intend to add but it isn't always a matter of trusting them with your line of credit. Sometimes it is more a case of where are they going and how security conscious are they?

They could be duped or careless with the security of the card and PIN. That might not leave you out of pocket if fraud occurs but it would leave you with a potential headache having to sort it all out. How careful are they? Are they a bit scatty and likely to lose the card and not notice for days? How savvy are they about shielding PIN entry and identifying dodgy establishments with dodgy staff etc?

The S75 issue alluded to by Willing2Learn is something to consider if they are likely to be making purchases that might benefit from such cover. I think most on the forum would agree that additional cardholder purchases are generally not covered by S75 CCA but there is some debate over cover if the additional cardholder is buying something at the express request of the main cardholder and where the main cardholder will be a beneficiary of that purchase. As the card is going to be used for travelling expenses it is less likely that S75 cover will be needed.

For my own experience, I had my wife as an additional cardholder on an account we used for joint expenditure for 30 years plus - no problem. Sadly, she now has dementia and has no concept of credit cards or shopping or pretty much anything at all any more.