"Is it a good idea to mix and match COVID-19 vaccines? Experts aren't so sure"
Barry Pakes does not have a safety concern per se but is concerned about efficacy being compromised. Later in the article Dr. Isaac Bogoch, though open to the idea, has a concern about safety and efficacy alike. Both experts admit that more research and data is needed.
Britain Opens Door to Mix-and-Match Vaccinations, Worrying Experts"
"Some scientists say Britain is gambling with its new guidance. “There are no data on this idea whatsoever,” said John Moore, a vaccine expert at Cornell University. Officials in Britain “seem to have abandoned science completely now and are just trying to guess their way out of a mess.”
CDC practically agrees here:
"Interchangeability with other COVID-19 vaccine products either of the currently authorized mRNA COVID-19 vaccines can be used when indicated; ACIP does not state a product preference. However, these mRNA COVID-19 vaccines are not interchangeable with each other or with other COVID-19 vaccine products. The safety and efficacy of a mixed-product series have not been evaluated. Both doses of the series should be completed with the same product."
If certain countries end up requiring certain vaccines for entry [assuming they make it mandatory] then it would complicate the matter all the more than it already is. It does not seem advisable.