The single issue I agree with is the lack of caching - installing NPM packages can take up to 15 minutes in the worst case. MS have known about this for years, so I honestly don't understand why they haven't done something about it.
I also think the hosted build machines are underpowered, despite their claims of Xeon CPUs - build on even an Azure B1ms VM (a low-powered, burstable SKU) are up to 3 times faster. In fact this is how we solve the NPM caching and performance issues - by running the VSTS Build Agent (it's OSS!) on relatively cheap Azure VMs.
Aside from these niggles though, I actually think the Azure DevOps product is fantastic.