If you haven't got their balance ... you have no chance of getting ikkyo. If you truly have their balance ... they have no way to counter.
100% of ikkyo problems are a lack of taking balance, or, uke regaining balance (due to tori error or uke craft) shortly after.
Which begs the question when is ikkyo ikkyo? For me the final pin has nothing to do with the technique. Oshitaoshi (Shodokan parlance) is all about catching and taking balance. Done with proper timing under the correct circumstance it is brutally effective and very very difficult to resist.
Done as a kneeling technique is the only time I practice the straight down and bury version. Here there really is no place for uke to escape to.