'value' is the annotation attribute that contains the comment text. Replacing it with something else won't work as you want to limit the length of the comment text in the output. Like Dominic said, the second parameter of elgg_get_excerpt takes the number of characters you want to limit the output to.

The default length for elgg_get_excerpt is 250 characters. You said you want the output to be consistent, so I did not change the default value of 250 characters in the code snippet I provided above because 250 characters is the number of characters also used in other type of river entries when only an excerpt is shown.

Dominic Milburn and Iionly - that solves the problem.
Next step will be to incorporate something so that unicode characters do not get broken at the end while shortening - there is a php snippet for this somewhere - I should find it out.
Also, it will be great if any of you have solution for this https://github.com/Elgg/Elgg/issues/5811 - if you can post there that issue can be closed and solved I guess.

Disha - You should be able to get round that bug by using view overriding system.  I think this bug is a low piroity becauase the core dev team are changing comments into objects.