He doesn't look good on film to me and I trust my eyes because I have actual boxing experience and knowledge inside and outside the ring.
In boxing like tennis it's how you win that creates the perception. Some fighters are spectacular performers. Mike Tyson for example, Roberto Duran destroying people. Their gifts are obvious speed and power forces of nature so to speak. It's exciting. Sugar Ray Robinson flash, dash and substance. Willie Pep defensive genius dancing, master, quite obvious. Then you have some fighters who are not spectacular performers. Their talent can be very understated. Monzon maybe one of them. Mike McCallum comes to mind. Bernard Hopkins perhaps Kenny Norton. The object is to win. To do whatever it takes to win to be well-rounded is a huge advantage. It's a fight! Style points don't really matter once the bell rings. For you basketball fans think Tim Duncan as opposed to Chris Webber. If you watch a highlight package of Chris Webber you would think he was much greater than Tim Duncan. Those of us that know basketball know that's not the case by a good margin. In tennis Ivan lendl as opposed to Jimmy Connors or John McEnroe. He was methodical effective and damn near unbeatable in his prime but he wasn't a spectacular performer the way perhaps Connors was at his peak. It didn't stop lendl from owning Connors ass though. Even the greatest fighter of all time which we all can agree was Rocky Marciano wasn't always the best looking guy in the room. Sometimes he wasn't that graceful. Sometimes he was downright clumsy, goofy, amateurish, not zesty, look easy to hit, wide open, ofcourse we know that was all by design. Sometimes people would tell Freddie Brown "hey Rock don't look so good in there" and Freddie Brown would say "see that guy laying over there on the canvas he don't look so good either"
I'm going to go out on a limb and say that probably a few of his opponents, a few of his challengers may have viewed his films as well and thought the same thing going in. Ask them after the fight they may give you a different opinion. Something that he's doing in there offset what they have.
Better examples in tennis would be Bjorn Borg or Mats Wilander in his early/mid career. Tho their serves improved as their careers went on both for the most part were like backboards, rallying endlessly until they got their points. Both had great endurance and footwork. If their opponents attacked the net they had pinpoint passing shots and lobs to repel them and when opponents (excepting Lendl a lot of times) tried to overpower them from the back court they run down a lot of shots always looking to make their opponent have to play that extra shot. Lendl when he came on the scene was the most powerful player ever seen and his combination of great shot velocity with plenty of topspin was unheard of. He had the greatest forehand in history at that point, a great serve and one of the best one handed topspin backhands ever. He ushered in the era of the powerhouse baseliner who still hit with great consistency. He admittedly was very dour and many would say boring, more in personality than ability tho.
I agree with this, and would add some of his most famous opponents were older or smaller men who moved up from their original weight class to meet Monzon. It's true. Monzon used consistently and grinding methodology to wear his less than dynamic opponents down. He never faced a man like Hagler, Toney, Hopkins, GGG, or Jones. Tables have turned these days on fans who used to say a lot of title defense means something as a primary reason for putting Mozon way up there. Many of the same people do not give active fighters similar credit. Can't have it both ways.
Monzon never faced off against a top natural middle weight with speed, or a top natural middle weight with big time power. Read the underline in bold type. Valdez was 5'9'1/2 tall, with a 69" reach. I believe he started his career at lightweight, hence he was far from a natural middle weight. He was on the slide when he fought Monzon anyway, but I do think we a good hitter even if his KO% is a lowly 59%. Indeed he floored Monzon, and might have retired him. Briscoe claims he was robbed in the first fight ( is there film ) And badly hurt Monzon in the re-match While Valdes and Briscoe were possibly Monzon best wins, they also show his limitations, and neither would make the cut as a top 20 all-time middleweight, in my opinion.
Followers of Show Ponies will never understand a true fighter like Monzon and continually look askance at his insane record as some sort of long term anomaly, the product of stars misaligning. I'm sure those same types prefer the flashy meaningless motions of a myriad of exposed combatants, precious pampered prodigies who never reached and held on to the brass ring but dissolved like sugar under the first trickle of adversity. Monzon was the farthest thing from a Show Pony. He was a man who worked toward victory with every movement and pause and had an unrelenting nose for the W. Not one of his combos were place holders or flippant but meaningful means to a meaningful end. It would sadden me that so many can not see a greatness so obviously proven in record and accomplishment. But then, sometimes true greatness needs a sliver of greatness in order to be recognized.
I don't think anyone here is saying he wasn't one of the greats, but rather they say he's overrated in the context of greatness. Monzon didn't fight great competition, and he sometimes struggled vs. the best he fought. 15 rounds suited King Calros, as he used his jab and size to control the action, slowly passing his mostly smaller opponents by in the later rounds like a top marathon runner separates himself from the pack.