I think this may have been this issue[0], which was fixed in [1]. Seems like the limit has crept up from 3 to 25 at some point in the last decade-ish, but in theory it should be undoable with 25X (or by mashing X to undo as many x commands as required).
[0]: https://github.com/philc/vimium/issues/1126
[1]: https://github.com/philc/vimium/pull/1128