Sometimes I'm just not sure how most people here complain about W10M freezing on their device and blaming Microsoft for it. You can blame Microsoft for the bugginess of the OS on their flagships, 950/XL, but from reading most of the user impressions here, WMPU forums, and in /r/windowsphone, most people who use the latest OS on phones other than the 950/XL generally have a good experience with it. Most of those who complains and rants about the supposed "bugginess" of the OS are using it on phones that in the first place were never meant to get that update anyway: Lumia devices with 512MB RAM. You'll be surprised to see how many people who posts complaints regarding the OS were actually using it on ancient phones ESPECIALLY ON THE 520.
The basis of "if the phone could handle WP8.1, it surely can handle W10M" is not valid. Heck, using my 620 for 2 years made me realize that as much as the WP OS is optimized for lower end phones, the hardware itself is bottle-necking the performance of the OS. Even my 620 on WP8.1 had lag issues, and I'm pretty sure if I updated it to W10M, I'll have a very bad and laggy experience. But the question I would've asked myself would be: Is the drawback in performance due to the software, or is it due to the hardware? Of course, my 620 came out of the box with WP8.0, and it was blazing smooth. But the OS at that time were really stripped down, with lots of features currently available on W10M and WP8.1 were not available back then (action centre, anyone?). After the 8.1 update, my phone's performance worsened. Lags were a norm, and battery life became worse. I really blamed the update for it at first, but after upgrading to my current 830, my eyes were opened. WP8.1 was really smooth on the 830, and the overall feel of the OS on my new phone is a big leap from what it was on my 620.
What I'm trying to say here is that, sometimes, we require better hardware to support new software. I personally just updated my 830 to W10M this week, and it's been a really smooth sail so far in terms of performance. Bugs were non-existant except for a few veeeery minor bugs that doesn't really affect the overall experience with the OS (ask me later if you want to know more about these bugs). The only downside to W10M for me are only the features of the core apps of the OS, such as Groove Music and the People Apps. Other than that, I really have nothing much to complain about.
But of course, I understand that some of you held on to your old Lumias due to your financial situation, and I respect that. But then again, you should also note that complaining about the instability of an OS when it's actually the hardware of your phone is bottlenecking it's performance, is really unfair and serves only to generate more negativity to an OS that is actually really good and very usable (it's on par with WP8.1 IMO).