Actually, there's nothing really wrong with Mega Houndoom. People bashing on it is usually out of little experience in using it. It is commonly regarded as a bad mega due to its low tier ranking and the fact that very few people actually use it, but in today's metagame, once it manages to Mega evolve and get to +2, it will bop everything slower then it.
Its defenses might seem sub-par but they are all it needs to function in today's metagame
At first sight, you could say that its stats in conjunction with its ability aren't that mindblowing, but they enable it to pack quite a punch. It's movepool is also rather interesting so when taking these stuff into consideration, you will realise that it really isn't as bad as people perceive it to be.
Due to the current state of the metagame, Mega Houndoom is actually a really good Pokemon. With fast threats like Greninja gone, its most reliable revenge killers are Scarfed Landorus-T, Keldeo and Azumarill (not always), Mega Altaria (defeated after one Nasty Plot) etc.
With the advent of new megas, Mega Houndoom can serve as an effective check to Mega Metagross and actually completely set up on the likes of Mega Sableye. Its excellent speed tier are also one of the many things that make it a rather interesting mega, especially in this current metagame.
To give you a better view on its power, I have provided a few calcs
+2 252 SpA Mega Houndoom Fire Blast vs. 252 HP / 252+ SpD Mega Sableye: 249-294 (81.9 - 96.7%) -- guaranteed 2HKO
+2 252 SpA Mega Houndoom Fire Blast vs. 252 HP / 0 SpD Mega Sableye: 337-397 (110.8 - 130.5%) -- guaranteed OHKO
+2 252 SpA Mega Houndoom Sludge Bomb vs. 252 HP / 96+ SpD Clefable: 372-438 (94.4 - 111.1%) -- 68.8% chance to OHKO
+2 252 SpA Mega Houndoom Sludge Bomb vs. 0 HP / 0- SpD Mega Diancie: 213-251 (88.3 - 104.1%) -- guaranteed OHKO after Stealth Rock
+2 252 SpA Mega Houndoom Sludge Bomb vs. 240 HP / 16 SpD Assault Vest Azumarill: 328-386 (81.7 - 96.2%) -- 56.3% chance to OHKO after
Obviously like all Pokemon, it isn't a flawless Pokemon. It usually needs to be at +2 to be doing some impressive damage and finding a good time to set up isn't always as easy, which is what many people have issues with when using Mega Houndoom, but it doesn't necessarily need to. Its offensive type coverage is of great value in every tier it can be used in ( bar ubers ofc).
Looking through the UU tier list alone exposes how many Pokemon will tend to grant it a chance to Mega evolve.
In UU, thee are a few Pokemon that can check it properly, with Tentacruel being a great one. With its enormous special bulk, it can tank multiple hits from Mega Houndoom and force it back out or threaten to severely damage it with its water stabs
Suicune works in a similar fashion, but then again, Suicune is a monster in OU so that shouldn't really surprise you.
Faster checks would be Aerodactly and Maerodactly. Due to their high speed, they naturally outpace Mega Houndoom and can easily revenge kill it, however they can't switch-in reliably.
Blissey sits on Mega Houndoom. There's nothing to elaborate on
Goodra's great special bulk also make it a good switch-in to Houndoom as it can do so multiple times, however should Mega Houndoom be carrying WoW, it can be worn down.
Snorlax's great bulk threaten Mega Houndoom, especially due to the fact that it can retaliate with Earthquakes and body slams.
Infernape's Mach Punches can pick off weakened Mega Houndoom.