A ballot’s potential for expression is only as good as the accuracy of the information the voter puts on it. Unfortunately, all deterministic voting methods give voters an incentive to put insincere (i.e. inaccurate) information on their ballots. This is true for both IRV and approval voting, but the tactical incentives with IRV are substantially more detrimental to its performance.
To explain why this is, the general tactic with virtually every deterministic voting method is to identify the two most likely winners, and give the more favored one as much support as possible, while giving the other as little support as possible.
With approval voting, that still leaves the voter free to approve everyone he sincerely prefers to the frontrunners, making it quite resistant to tactical behavior. Whereas this is not possible with any voting method which forbids giving equal support to multiple candidates. Hence those methods, particularly IRV, tend to degenerate toward plurality voting in practice.