How risky is it to get tickets in Florence to the Accademia or Uffizi museum the day of trip on Sunday in April 28?
What do you mean by risky?
You'll be standing in some long line probably.
If you mean risky as in "can I get tickets?" Yes, you can probably get tickets, but, as noted, you will likely stand in a very very long line, for perhaps hours, at each museum. Book ahead and get timed-entry tickets if you have other things you might want to do in Florence that day.