if a unit is within a the blast radius is always considered a hit?
No.
For example, if between you and the blast there is something that blocks the blast (e.g. big wall), it's not a hit.
Sometimes i see a projectile weapon with a bulletsprite hiting the unit(visually) and not count as hit (on the hit log)!
I would need to see it... there's a million things that can go wrong.
But in general, if you're clearly hitting... there should also be an entry in the hit log.
or does it count as a hit (on the hit log says 0 (miss)) and the roll as 0% (on the 0-200% UFOdamageformula)
0 is not a miss, 0 is a hit
why the question ? to be sure the explosion always destroy tiles always 100% of the time if i put the roll at 100%-100%
How does this relate to anything before?
Hitting tiles/terrain and hitting units are two completely different things... the hit log does NOT show if you hit the tiles/terrain or not.