can i get some feedback on how strong fillet brazing is?

That's a really cool frame.

like that one, extremely.
I have wanted one for a bit.

Fillet brazing is strong if done well.

that's going to be a very stiff frame.

A properly executed fillet braze is stronger than the tubes themselves, so no worries there.
I'd be more concerned with what looks like a gusset between the top and down tubes, is that a plate? Looks cool, but a gusset there (on the tube centerline) can actually create stress points...I hope the tubes are pretty thick to compensate for that.

that makino is a keirin frame, built to be light (in more ways than going fillet instead of lugged). if you're asking about its strength, you either got your answer in comments above, or you plan on using this bike for tricks. PLEASE for the love of all great keirin framebuilders do nont go crazy bmx on this frame. the top tube, for example will dent easily compared to, lets say, a Volume Cutter, because the makino tubes (esp. top tubes with keirin frames) were likely as thin as possible to maximize low frame weight. it'd be like doing motocross on a ducati or something.