What are disadvantages of the Hadoop distribution MapR compared to Cloudera and Hortonworks? [closed]

I would define MapR a bit differently. It does not use HDFS, but instead of it provides their own distributed file system with NFS interface. which, as well as HDFS is based on local FS.
Main differances are coming from the fact that HDFS is not Posix and other design choices.
1. HDFS is not mutable while MapR is. It can be viewed as advantage, especially if you need it.
2. HDFS is not mountable while MapR is. You can use any existing tools working with Linux FS.

Unrelated to posix:MapR have small block size and not single point of failure (NameNode).MapR Has multisite replication.

lets look on dark side also:a) Having mutable data (instead of not mutable HDFS) makes system more complicated.
b) It is not known (at least for me) to work on huge clusters. (I heard about hundred of nodes).
c) From architecture point (having small blocks) I am not sure how good data locality can be achieved.

David, the minute-sort record was set by MapR on the Google Compute Engine in the Google Cloud on 1/30/2013. See our blog at http://www.mapr.com/blog/hadoop-minutesort-record. The record was set on a 2103-node cluster and 1.5 TB of data was sorted in 59 seconds.

Also see an earlier blog about the Terasort record by MapR sorting 1 TB of data in 54 seconds.It was set on a 1003-node cluster on the Google Compute Engine in the Google Cloud. The blog is posted at http://www.mapr.com/blog/record-setting-hadoop-in-the-cloud.

Also see answers.mapr.com for many questions/answers on this topic.

Until some impartial source does extensive benchmarking (under varying workloads) of Apache Hadoop vs. MapR's version, I think we cannot categorically say one is faster than the other. If records are going to determine your opinion, then you should now that the current terasort record is held by Yahoo, with Apache Hadoop. Details here and here.