I don't think it's completely irrelevant. Acknowledging that a character has a particular design goal and a role that the character fulfills doesn't mean you should ignore the character's weaknesses as part of his design. In Ephraim's case, he has the stats of a more mobile tank without the skills. Part of why Effie is so good is that she has Wary Guard. Since Ephraim doesn't have that, you have to consider how that affects his current and potential matchups.
Perhaps I didn't explain it properly. Ephraim's role is to be a tank buster so naturally he'll go up against tanks frequently. Tanks will most likely have low speed. As a result, Ephraim's supposed "weakness" is not really a weakness. That being said is his speed lower when compared to many other units? Yes, but he's not necessarily designed to take down those encounters. If he was then he'd be considered OP and really good. I guess this is what you're trying to get at? Weaknesses exist with Ephraim yes but the weakness is irrelevant when considering his role. Weakness and irrelevancy in this situation are mutually exclusive.