What are IOT indexes and what are all the disadvantages for this?

IOT are Tables organized as Indexes

The advantage is it lets you organize a table in the order most commonly accessed, and lets you combine storage for a heap table + primary key.

A typical heap table in Oracle has no ordering. Then you create a primary key index which is stored in a 2nd segment. So you have to segments of data now. Also you have redundant storage of the indexed columns. So when querying by the primary key, Oracle has to first access the index, then the table in most cases.

An IOT gets rid of the redundancy, and also orders the rows in the actual table, because it is an index, with the additional difference that the IOT also has the full data for each row stored in it, not just the keys.

The disadvantage would be only in the case of access of the IOT by a key other than the primary key, that requires a secondary index, but the difference is, on an IOT, secondary indexes use logical rowids, not physical, so there is an extra layer of abstraction when accessing the rows.

There are some other minor constraints as well, in the docs, but most are special case issues.

In general, IOTs work very well for tables where most of the access is either unsorted scans, scans sorted by key, or random access by primary key.

I raised similar question few months ago, it might help you

https://www.experts-exchange.com/questions/26636850/INDEX-and-INDEX-ORGANIZED-TABLE.html

You can always create a table, then an index of all its columns.  This way it is easier to drop and rebuild indexes if you need to.