I got a super-squinter barely there line on these @ 10DPO which made me immediately retest with a FRER (nice clear line) and a Clearblue digi (pregnant 1-2) which are both 25ml/iu!!!!!!
And yes, they take AGES to get dark
I think the benefit of them, is that you don't use the expensive tests willy-nilly, so good for economy!! But I feel you'd always need to confirm with a different brand