The Portland Timbers are a decent MLS side, with decent playoff history. They make the playoffs fairly often, even won a title. What they are not, is a good MLS Playoffs side, or a good USL Playoffs side. Their latest coach may be a good playoffs coach, but has never been tested over more than two games in a single year (Sunday will be his second playoff game of 2018).
Sure, they beat the Sounders in the only MLS Cup Playoffs meeting. They have not beaten Brian Schmetzer in a playoff series (Schmetzer is 3-1-0, +3 in their USL playoff meetings).