EDIT

Strange is that on some systems this is only logged as warning while others do throw an exception. I did not yet find out if/how this can be configured in MySQL.
Probably it is only a problem on ISAM as InnoDB allows larger indexes if innodb_large_prefix is enabled.

However, the problem can be solved by using a prefix a column prefix key part (https://dev.mysql.com/doc/refman/8.0/en/create-index.html#create-index-column-prefixes).
But the problem is that hibernate in the current version does not support this.
Therefore I created a new IndexExporter (MySqlIsamIndexExporter) which extends the StandardIndexExporter and which is used by the MySQL5MyISAMUtf8Dialect.

This seems to solve the problem.

Only open issues are

• adapt the uniqueKey on AuthorityBase accordingly which spans 4 columns and is a uniquekey => done in #8464
• make it possible to activate holdOnError flag in SchemaExport to allow throwing Exceptions if problems occur during SchemaExport, otherwise Users don't get informed in cretain contexts
  
• (related) create schema ddl for tests

make it possible to activate holdOnError flag in SchemaExport to allow throwing Exceptions if problems occur during SchemaExport, otherwise Users don't get informed in certain contexts

This seems quite complicated since the SessionFactory is a final class, but you could try something else:

Create a looging appender org.apache.log4j.Appender for the logging framework being in use, currently this is log4j and register this apppender. The appender will receive the logging event issued via LOG.unsuccessfulCreate( sqlCommand ); from the logger created as CoreLogging.messageLogger( SchemaExport.class );. The appender could directly notify the UI when an according LogginEvent is coming in:

SomeEventBus uiEventBus = getUiEventBus();
Logger logger = Logger.getLogger(SchemaExport.class);
logger.addAppender(new AppenderSkeleton() {

    @Override
    public boolean requiresLayout() {
        return false;
    }

    @Override
    public void close() {
        // TODO Auto-generated method stub

    }

    @Override
    protected void append(LoggingEvent event) {
        if(event.getMessage().toString().startsWith("Unsuccessful: create index")) {
            uiEventBus.notify(event.getMessage().toString());               
        }

    }
});

I copied the exception handling to a new ticket #9086.

I think the real issue of this ticket (exceptions are thrown in MySQL schema generation) should be solved. Do you agree?

yes in deed!