They have the lowest fees but offer funds from one firm, themselves. If you only want to invest in Vanguard products thats fine and go Vanguard, however if you want anything else you have to look at other platforms.
You can't access rest of market or single line stock so it is very limiting
If you use ii/interactive investor you get low fees and you get to invest in Vanguard and most of the other things out there. Nutmeg was super simple but I figured with a little more time on my part reading I could save myself coin in the long run.
Imo Vanguard is good if you want to invest in Vanguard funds
Nutmeg is simple and good for basic model portfolios
For whole of market Hargreaves Lansdowne is worth checking out as a platform.