Cleveland's most recent performance in the playoffs should be extremely relevant to any projections to how they'll fare in the Finals. A month and a half and three series' worth of play is a great sample size for the playoffs given that there's only a single round and up to seven games remaining.
The playoffs, though, are so matchup dependent. The Cavs have looked good over three series, but they also have not been seriously challenged in any of their matchups to this point.
You're right to point out that the Warriors haven't exactly faced the kind of gauntlet that, say, the 2013 Spurs faced on their way to the Finals.
Still, to me the playoffs is all about the best players on each team in each series going head to head, and the Cavs simply haven't faced a single team that has had a single guy who could hope to go toe-to-toe with LeBron over the course of a seven game series, in the way that Paul Pierce, Dwight Howard, Dirk Nowitzki, Rajon Rondo, and Kawhi Leonard have in the past. To me, that means they are untested, despite being in the Finals.