Outside of last seasons there is some truth to that. Jax has great peripherals, the second best FIP of all pitchers with at least 65 IP the last 3 seasons, and he has come in and helped his team a lot, 2nd in Shut Downs (appearances in which a RP increases their team's win percentage 5%+). But he also has the second most Melt Downs (win percentage for team decreased 5%+).
I would suggest Brock Stewart is completely unreliable though and should be traded today. No team should count on his long term health, but him being healthy now makes him enticing to contenders I would think.