Agreed, synchrons are a fun casual deck, which can catch a few decks out (can be a low tier rogue deck if you play around with it- personal experience is good against virtual world, dragon link, adamantia and (pure) zoodiacs.

Synchrons biggest weakness is going 2nd (hand traps not so much) as it can lose hard against decks which prohibit extra deck summons and going 2nd against eldilich & its variants / dragoon decks (if forbidden droplet or dark ruler no more are not in hand) and certain decks with restrictions lifted e.g. skystrikers with 3 multirole, engage and widow anchor).

I think konami were just scared of synchro spam when MR5 came out as junk speeder is in the ed zone and it gives the option of bringing out a levels 5 (if you want ), a 4,3,2,1 synchro if you want. if you summon with junk converter, you can bring back a tuner from your grave with no extra cost. use doppell warrior, you get 2 level 1 tokens. You have synchron carrier, any tuner and junk converter in your hand, you can easily bring out a few ace monsters.

Irony is, newer synchrons and dawn of majesty help junk speeder a lot. stardust synchron and arriving miracle allow extra draw and special summon options. have brotaur in the deck and you have longevity in play. get an bad or average hand, you can stall a tiny bit or summon a level 7 or 8 synchro monster or a halqifibrax combo . get a good or great hand, you can get 2-3 monsters with 4000 or more atk with mass negates in turn 1