It seems since they used all cavalry armies they would be very mobile but lack standing power. Cavalry is bad at defense. How did the Mongols held dedicated enemy attacks at bay? Feigning for long periods may cause the enemy to attack your camp (as happened in manzikert) and cause loss of precious resources for the campaign.
They didn't use only cavalry. A big part of why the mongols were so successful is their willingness to incorporate tactics and technologies of conquered civilizations into their own armies.
While they did use a ton of cavalry, and were extremely successful in doing so, they also used infantry and had plenty of mercenaries fighting for them.