Like many things, the devil is in the details. The carrier oil makes a big difference, as does the amount injected, where injected, any doctor you may be using and the country you live in. Not to mention, insurance if you are using it.
When you talk about Testosterone Undecanoate and 21 day half life, you are implying Testosterone Undecanoate in tea seed oil which mainly comes from China and is (I believe) generic.
A few years ago I looking into it, it's probably manufactured OK, but it's not a brand name (Bayer, etc) so IMO it "could be" a dicey product. It was pretty cheap and came in 10 ml vials.
Nebido is 4 ml Testosterone Undecanoate in castor oil, and has a half life of 33.9 days. Aveed is the USA version with 33.9 days half life, 3 ml and a shorter injection schedule.
I only use Nebido, which I prefer over Test-C or creams/gels, though I have only tried the creams and not testosterone cypionate, though I have a couple of vials as "backup".
The negatives about Nebido is you need to inject in the glutes and it's harder to come by if you live in the USA as you would need to import it from a country where it is approved. Aveed is negative because the US manufacturer significantly boosted the price for USA Aveed version for US consumers and it's only 3 ml vs 4 ml. If you enjoy dialing in and fooling with dosages, Nebido wouldn't be any fun since you just inject the same amount once every 10-14 weeks, the only variation is the 10-14 week window.
The positives of Nebido is only injecting 10-14 weeks and lower HCT. The highest my HCT has been is 47.5 and the highest I have measured my total T after 17 days was north of 1500, but only saw that once. The lowest I have seen is 541 after 75 days.
I suspect whatever you start off with and find easy to get is the "best" choice for most people. I may have to switch to testosterone cypionate as it recently became more difficult to get Nebido.
When you say inject twice a month, it depends on how much you are injecting and what the carrier oil was. As far as I am aware of, there is no standard protocol for generic Testosterone Undecanoate (not Nebido) so you are on your own to experiment, which can be a negative or positive depending on your desire to experiment.