I reckon there are at least two big problems with Devo-max or the devolution of more powers as an option. One, the shower of shite sat in Holyrood and two, it would leave the chip on the Scottish shoulder firmly in place.
Re: One – Devo-max would simply give more power to the existing MSPs. There would be no gradual transformation of Scottish politics wherein Holyrood became an end in itself as opposed to the ex-councillors' gravy train it currently is. Plus, the ongoing tension between what UK level political parties want and what their Scottish offshoots do would remain. To be fair this second point is primarily a Labour party issue given what the Tories do here is largely irrelevant as are the Greens and as for the SNP, well the clue’s in the title.
Re: Two – Devo-max would leave the blame England mentality in place, something both supporters and opponents of devolution have been known to buy into like perpetual adolescents.
The two problems are inextricably inter-linked; the key issues for a devolved government are getting and spending, by which I mean periodically negotiating (UK) central government funding levels, then apportioning said monies out across the various devolved areas of responsibilities.
And just as every council blames central government funding cuts whenever it cuts services, so a devolved Scottish parliament can blame Westminster when it fails to get what it wants out of negotiations or just for the heck of it.
What a devolved Scottish politics, as with any adolescent, doesn’t do is take full responsibility for the consequences of its actions. Remember, Holyrood already had the power to vary income tax, just not the self-confidence or political will to do so.