Let's face it. The budget phones of early 2010s were horrible. You may have good memories of them, you may have gotten your first smartphone back then, but they were objectively much worse than flagships of that era. They compromised on everything - performance, camera, build quality, etc.
So naturally, it was the case for many years that the only option for getting a smartphone with decent specs on a budget, was buying an older flagship.
The situation changed in the past 2-3 years. We have many brands like Xiaomi, Meizu or ZTE that focus on budget devices. The budget propositions from more familiar smartphone manufacturers like HTC or LG also got much more reasonable. So, is buying an older flagship still a reasonable choice? Let's take look at advantages and disadvantages of them.
Advantages:
Premium build quality.
There weren't pretty much any technological advancements in materials from which the smartphones are made. So if you want your smartphone to be made out of the top quality glass and aluminium instead of plastic, you definitely should consider buying an older flagship.
Camera is always decent.
It is true that there are some camera-focused budget smartphones that will perform better than 2-years old flagships. However, in most cases, camera of budget smartphones is still hit and miss. You need to do a lot of research and in many cases assess quality of pictures visually, as those smartphones won't be featured in any rankings. With flagships on the other hand, you can be sure that if their cameras were the best 2 years ago, they will still be pretty good today.
If you go with bigger brand, there are no problems with warranty.
As explained in this article https://steemit.com/hardware/@hwtrendsetter/claiming-warranty-in-china-actually-not-a-bad-idea it is possible to claim warranty on all Chinese phones. However, it's not quite as simple as walking into the nearest Sony Center, leaving your phone with them and getting a text 3 days later that it has been repaired.
All of them come with NFC.
One of the features that designers of most budget smartphones compromise on is the NFC module for mobile payments. For many people it's just a gimmick. Others don't need it in smartphones because they use NFC in their smart watches. But for some people it's a deal breaker. So if you use NFC in your smartphone and are on a budget, the best way is to go with an older flagship.
Disadvantages:
They are not quite as power efficient as the best budget phones from 2018.
As a result of modernized technological process, modern smartphone CPUs don't require as much energy to power them as their older counterparts. And while in terms of performance they will in most cases be similar, in terms of power consumption (resulting in shorter battery life) flagships are more likely to lose.
Lack of software updates.
If you are buying an older flagship after 2 years since its release, the support has most likely already stopped and you will be stuck with Android 6 (or Android 7 if you're lucky). That's still definitely usable today, but who knows how the situation will look like in 2 years. Whereas if you go with a budget device from 2018, it will not only be starting with Android 8, but it will be probably getting updates at least for a year (or in case of Xiaomi and Meizu for many years).
You don't get the latest features.
Face unlock, dual camera with bokeh effect, AI selfies, 18:9 aspect ratio, gesture controls. If you want any of those recently developed features, you cannot go with an older flagship.
They don't follow the latest trends in design.
It doesn't matter if you buy a phone from 2018 for 100€ or for 1000€, they all look like (at least to some extent) iPhone X. So if you go with older flagship, your friends may be wondering if you really bought a new phone, or you are just reusing your older brother's old Nokia.
To sum up, buying an older flagship is still a viable alternative in 2018. Especially if you're not sure what you're doing, and don't want to end up with a smartphone that would compromise on something too much. However, if you want to take advantage of the latest innovations like face unlock or portrait mode with bokeh effect, you will need to look into modern budget phones.
Unfortunately, you cannot have everything for 150€, so if you want the latest features, and NFC, and premium build quality, you will need to spend more.
Take a look at few examples of once the latest and the greatest phones, now being sold for below 150€:
HTC 10 EVO 135€
https://www.ebay.de/itm/273411823452Sony Xperia XZ 144.99€
https://www.ebay.de/b/Apple-Handys-Smartphones-in-Silber/9355/bn_471721Huawei P9 149.99€
https://www.ebay.de/p/221938545?iid=323406013520