I don't think that there is a complete 'tool' out there to really capture platform features and costs in one comparison. You tend to get one or the other, or a bit of both without additional information that might be pertinent to the client being advised. For example, there aren't many platforms that allow investment phasing unless it is over a certain period of time. I'd be surprised if any comparison tool showed that up.
The Platforum and the Lang Cat produce great glossy documents for features and overviews of platforms but there is no tool to go with them, and the example cases they put into the documents are very specific so cannot be used in most cases. I've also found errors with some of the more detailed research, so the advice is never to rely on them and still carry out your own (goes without saying though I guess!)
I haven't used adviser asset since they started charging for the service, but maybe someone else could give an opinion on that. That might be worth looking at.
If you are carrying out regular due diligence I would suggest building your own tool in MS Excel or similar software. You'll find the things that are bespoke to your advisers/company requirements (there are always some things!) can be tracked and updated better that way. Best example off the top of my head is if you use in house model portfolios.
We have a macro- enabled spreadsheet for our due diligence; if a client needs something in particular there is a box to tick and those unable to provide the feature disappear. The cost comparison can then be undertaken on the rest and the file PDF'ed and saved to the client.
Late to the party but looking to launch a new platform due diligence tool and consultancy business later this year - watch this space and feel free to message if you would like any initial info.