It's possible it's not MT and they just didn't write the facade APIs. That'd be pretty crazy.
My biggest complaint would be lack of plugin support.
Also of note: Amazon's documentation on HTTP limits is wrong. There are some instance types listed as having a 100mb max payload that are only 10mb. We found that out when Logstash recorded a crapload of errors with the 10mb limit on what was allegedly a 100mb instance type.
It depends on your use case. If you are already familiar with Solr and it is good enough for your use case, then use it. Solr and ES are about the same feature-wise. Scaling is easier for ES because it is built-in. Here is a good comparison of their APIs.
part 1: http://opensourceconnections.com/blog/2015/12/15/solr-vs-ela...
part 2: http://opensourceconnections.com/blog/2016/01/22/solr-vs-ela...