That is not at all the case with T-Mobile and Google Voice. You do first have to activate your T-Mobile voicemail to keep the voicemail notification from popping up each time your phone is rebooted, but after that, you can choose Google Voice as your voicemail service in the Phone->Settings ->Call settings->Voicemail (or, just let the Google Voice setup wizard do it for you). You can also put your Google Voice number in the Phone->Settings->Call forwarding settings for "Forward when busy", "Forward when unanswered", and "Forward when unreachable".
As far as international use is concerned with T-Mobile's Simple Choice plans, yes, as of November 1, you get free 2G international data roaming. Texting with US/Canadian numbers is always free no matter where you are with T-Mobile's international roaming, too (btw, texting with Canadian numbers is free on all 4 major US carriers both while you are in the US and in Canada, MMS is considered 3G data, though, so that costs money if you're in Canada). You can also send/receive texts to international numbers (non-"+1-xxx-xxx-xxxx" numbers) for free, however, those users may be charged per their phone provider's rules.
You can also use T-Mobile's WiFi calling while overseas to make calls (the T-Mobile store told me that when I was activating my new account with them), however, since they do not install any software on the Nexus 5, you have to get it setup somewhere that I have not yet found the settings for (I didn't need to make calls on my last trip to Canada, so I didn't bother looking very hard. You get unlimited minutes on Simple Choice plans, so why waste data for calls while you are in the US unless you just don't have cell signal and need to connect to WiFi, instead?). It's not "removed" as it is not a part of the Nexus software, you just have to add in the settings, which I'd imagine is in the Internet Call Settings section of the Phone->Settings->Call settings.