Having disassembled the aforementioned Z3C a couple of times, I disagree that it's fine to have to break out the tools for it ever. It's a tough job, with lovely warnings like this one from iFixit:

"Warning first: Never try to move the battery unless it's broken and need to replace. Because there is very very strong adhesive sticker between battery and motherboard.And too much effort or careless operation will break the motherboard."

It's also difficult to maintain the waterproof seal when reassembling the phone. So while the Z3C has great battery life, you're essentially only going to have that for a couple of years.

Funnily enough, for all the bad press Apple gets for glued together devices, the battery is trivially easy to replace in iPhones.

This is, I suspect, a byproduct of the fact that they handle (the vast majority of) their own frontline customer support and repairs. The most commonly-repaired parts are designed to make the process as efficient as possible for the Apple Store techs.

That's exactly it. Apple Retail iPhone 'techs' are relatively low skilled and low paid, and have a very small amount of time to do what they do. The most common parts, like camera, speaker, battery, screen etc. are made like Lego to make it easier and quicker for them to do their job.

The phone isn't quite the same once an unprofessional like me has opened it without guidance. For instance this nexus 5 on which I replaced a cracked LCD now has a little bit of give when I press the back. It used to feel completely solid before.

Yeah, this is because there's a sealing tape that's supposed to go to the frame after opening.

Original frames usually have it attached to the frame, aftermarket ones don't. Some tape or glue needed to close the original one after opening.

While it is of course possible to replace the battery in most phones with specialized tools, for some of the newer models it's very difficult to do without damaging the device. When we're talking about a phone you just bought on eBay with the intention of making it your daily driver, that's a hard sell.

All other things equal, I will always pick the slightly thicker phone with a removable battery over the one that might have me breaking out the spudger.

The specialized tools are a tiny screwdriver and a plastic pry knife on some phones - the adhesive doesn't come in until screen replacement. You can buy battery replacement kits with that stuff included.