Here are some of my experiences of using XML to export from Sibelius and then import into Dorico. These oddities don’t happen every time - as eddjcaine says, it depends on the complexity and size of your scores.
Normal ties, particularly in cues, convert to laissez vibrer ties;
Cues convert to normal size notes;
Separate voices on one staff get swapped around or sometimes combined into one voice;
Tuplets can get very badly mangled and, in the worst cases, have prevented any further notes from being imported on that staff;
Beaming and note grouping are not as in the original;
Text of various sorts (lyrics, tempo, dynamics, instrument names, etc.) imports as plain text or is allocated seemingly at random to the wrong category of text.
I don’t know whether the problems occur because of how Sibelius exports the XML information or how Dorico interprets it on import, or maybe a bit of both, but I have examined some of the XML files I have exported from Sibelius and found errors and omissions. I have noticed that the problems occur much more often if I have had to utilise fancy workarounds to overcome some of the shortcomings in Sibelius, particularly with tuplets.
On average, I have found XML very handy, but I always need to check the file after it has been imported into Dorico.