It's really simple. Jim Mather intentionally told the people of Dunoon what they wanted to hear. That's how he was able to boot out the Lib Dem MSP, and gain office.
Mr Mather, and the SNP, should be aware that he wouldn't have been elected had he not made that promise. The SNP won by one seat, and are now using that victory to claim legitimacy in railroading their plans to leave the UK. Maybe it's time that they started paying attention to the things that actually got them elected.
Without our votes, given on the back of this promise, Mr Mather wouldn't have a job as an MSP and Mr Salmond wouldn't have a job as First Minister.
As for my views on "other things" he has done I'm a little unsure of any. Perhaps looking at his role as a Minister of a government that promised to:
1. Scrap student debt
2. Introduce first time homeowner grants
3. Recruit 1000 extra police (which they tried not to do until the tories forced them into it)
4. Reduce class sizes to 18
Jim Mather has betrayed the people of Argyll and Bute as an MSP and he has betrayed the people of Scotland as a Minister. There's still time for him to turn around and start to try and keep to the promises that he used to win our votes, if he does that he might have a bit of hope, if not any credibility. If not I imagine we shall be soundly voting to remove him from office come the next election.