I'd say the major downside is that it kills the flavour (as do all 510s)
& you're lucky to have gotten 5.5 hours out of it. The most I have ever got out of a Mega is just under 4 hours (but this is indeed twice what you'd get out of a normal Titan battery, can't comment on the battery life of other 510s as I haven't got any & don't recall reading any comment on them). This is on par with my worst M401 (I can get 5 - 6 hours out of the EVOs & EM4s), in fact I have a couple of M402 EVO batteries that will give 4 hours. (I am quite a heavy vaper, I barely put it down)
I don't have any complaints about the 510 flavor, but then I can only compare to the GreenSmoke, which I do not like the flavor of at all, no matter the cart flavor. The time-extension of the mega batts for me definitely is worth the little bit extra in price. I have heard that the 510 runs higher voltage than some of the other "normal" PVs, and maybe this kills flavor some? IDK. I actually use a passthrough the majority of the time, so the point is largely moot.
I make my own flavored juices though from Lorann, VG and high-nic juice PG/VG juice. So if I want more flavor I just increase its % in the mix. I only have a few commercially flavored juices, and they are all with PG only. I think VG helps bring out the flavors. Maybe I'll get one of the models you mention in the future, but I am blissfully happy with my setup now. I've heard good things about the EV, but I'm not fond of PVs that look like analogs. Looking to get some wine colored 510 attys and batts...nice for Fall colors. Simple elegance that says no, I am NOT smoking a cigarette, this is cooler and way better. But to each his own, and hte bottom line is the more variety of hardware that is available, the more chances of long-term post-ban survival we will have.
Vape like and ape with a cape!!
Kurt