He whitewashed the best team of his era (no.1 England of 2012 that beat India in India) in a neutral venue. Then beat them again last year.
England are again arguably the best Test side in the world, it will be a big challenge to remain undefeated at their home.
Plus, it's also huge credit to him that we're virtually a fortress at a neutral venue. In fact, no other Pakistani team has ever been so successful even at our 'HOME' Pakistan. We've always had teams defeating us at our home.
Therein lies your answer. Pakistan has had much better results at the UAE than even at its home. So basically there are three options - either the yesteryear Pakistan teams were so average that they couldn't dominate at home, or the current team is so strong that it can dominate even away from home or it is generally easier for the Pakistan team to win at the UAE when compared to its home. You take your pick.
It is not about beating England at the UAE. Let's be honest, Pakistan has been playing at the UAE for long enough to consider it as the home venue for Pakistan. Well yeah technically it is termed as "neutral", but even you would agree that the UAE conditions are more favourable to Pakistan than it is to a team like England or Australia or NZ. I think the undefeated record and the fortress thing is very commendable and not many teams can hold that but it is also a bit overrated here. I mean Pakistan are not a minnow to celebrate an undefeated record. Don't get me wrong, that record is very good but top teams are "expected" to win at home. Even if Australia has a similar record but it draws series at home, it would be terribly disappointed for having drawn those series at home. Still, it's a very good record in itself and Misbah should definitely be lauded for that.
However the reason Misbah's test record is not held in the same regard as Imran's is because Imran's team was very competitive against top teams away from home. Compare that to Misbah's, Pakistan has played the following teams away from Asia under Misbah:
New Zealand - a series win. A good achievement but it was a weak kiwi team before it transformed as a strong side. India won against the same wea side about an year before Pak did but was beaten when they toured next against a much stronger kiwi side (which drew against Pakistan at the UAE).
West Indies - Drawn series against an average at best windies side.
Zimbabwe - A drawn series with a loss against a poor Zim side (apart from the solitary test win before the series).
South Africa - This was probably the only top team that Pakistan faced under Misbah overseas amd was whitewashed there.
So basically regardless of the results at the UAE, Pakistan have come up short when they faced a top team and slipped against average and poor teams as well. Misbah has a lot to prove in the coming series and he will definitely need a series win in England and atleast a drawn series at Australia to be mentioned in the same sentence as Imran.