One of my players was questioning about Senechal Seeker of Lore special ability. According to him he understand that his ability would allow him to have success in any Cypher, Lore or Logic test, even if he does not have the appropriate skill , at the expense of one fate point.

I told him i disagree because the rule clearly states that you can a success in a test, which implies that he need to be able to roll. Since most lore skills and cyphers are advanced skills he can only roll on those if is trained on them, thus he can only spend a fate point to have success if he can try the roll.

On other hand he thinks if the skill is like i just described it would be weak in comparison to the rest of the skills from other classes, which i disagree.

What is the general opnion on this topic?

I would say your interpretation is correct. Your can't succeed, or fail, a test unless you can take it in the first place.

The power of Seeker of Lore depends on how often, and how important, lore tests are needed that your party cannot take their time with. If they don't come up, which is probably the case, then Seeker of Lore will not be particularly useful. How powerful the skill is is in large part up to you.

That being said, it would be improper to look at balance in terms of caree skills versus other career skills. The Seneschal has access to a number of skills that other careers do not, or at least do not have access to as early as the Senny. The Seneschal also has the best experience cost spread for characteristic advances, with only two super-expensive stats, one of which being quite unimportant for the Seneschal (strength). The Seneschal, in many ways, is by design able to plug holes in the rogue trader's diplomatic and commercial skillset while offering some other useful but not terribly necessary skills. If your party's RT is focused on trade and negotiations, a senseschal will be of fairly limited utility, while a more millitant RT will benefit greatly from a skilled seneschal.