This kind of なんだけど can be understood as a contraction of "A(しておい)て(いう/するのも)なんだけど", roughly meaning "it is awkward/inconsistent/contradictory etc. to say/do after A"
Hopefully the examples explain:
- 食べきっておいていうのもなんだけど、おいしくはない。 It is awkward to say after finishing it, but it was not really delicious.
- 来てもらってなんだけど、あんまり手伝ってもらうことはない I appreciate that you have come, but a bit awkward to say, there is not much to do here.
- 自分で言うのもなんだけど、これは完璧だ It is not right to say this about what I did myself, but this is perfect. (In this case, there is no A of the pattern)
I don't think the translations exactly convey the nuances of the Japanese sentences, but basically なんだけど indicates the situation/action of the speaker is not very consistent with what she says/does.
たいがい means basically "overall" or "most of the time". たいがいX is usually with a word X of negative connotation. For example, たいがいいい加減だ means someone is いい加減 most of the time, if not always. So in this sense たいがい makes the description softer.
So the bold part means it may be silly to say this after having used it for such a long time, but the tool is rather fishy.
Note that なんだけど itself appears more "normally".
- 明日のことなんだけど Regarding tomorrow('s schedule)
- 子供が病気なんだけど My kid is ill, so...