Can you give a few examples on which Pokémon you find to be successful and unsuccessful in that regard?
I personally love how Magmortar looks. It has that smug look on its face because it knows it's strong and can vaporize the opposition with its extremely powerful Fire-type moves.
An example I personally find to be very well done is Yanmega. It's a great evolution to Yanma, because it actually looks like something primordial (it's based off of an ancient, long-extinct species of dragonfly, Megaraptora, no less), which explains the evolution triggered by learning Ancient Power.
Instead of just giving examples, I'll go just go down the line and list why I like or dislike new final/mega evolutions (split evolutions don't count)
Steelix-pretty much an onix but metal. Simple, but isn't recognizably better
Pinsir-a much more drastic change, being unique and seperate from Scizor. The problem is it feels more like a counterpart than an evolution of Scizor, and that bugs me.
Kingdra-A little bigger, but more streamlined and aggressive. It looks bigger and stronger and faster, so I like this one. More importantly, it felt necessary because looking at it, Seadra does not look like a fully evolved pokemon.
Roserade/Ambipom/Weavile/Probobass/Tangrowth/Yamna/Gliscor/Mamoswine-These were just fillers in the dex. No they did not appear to be fully evolved, but the fact that they were in the same company as Sableye/Mawile/Volbeat/Illumise/Spinda/Castform/Kecleon/Luvdisc made this not a big deal. By emphasizing that these 6 could evolve further makes me wonder what's wrong with the other 8. This was remedied slightly by the first two getting megas, but I feel like GF just got lazy with the other 6.
That said, I like all their designs fine (except 'stach man), and they feel like a natural progression of their preevlolutions, so no big complaints.
Magnezone-A clever way to give Magneton, a less than clever evolution in itself, a new form, but still a good design in itself.
Lickilicky-Not an improvement on the design at all, why was this necessary?
Rhyperior-You took a rhino and turned it into a mountain with a drill for a nose and arms that can shoot rocks. Perfect example of trying too hard. Not my least favorite at all, but why?
Electivire-A japanese ogre got bigger and hairier. Not a bad execution, but I can't talk about this line without griping about how little sense Elekid makes. Electivire's not bad though.
Magmortar-Where did the cannons come from? Where'd its beak go? Why'd you make it so fat? Where'd 2 of its fingers go? Why'd it grow shoulder pads? Why does that smile look s creepy (reminds me of those clowns people hate)? All in all, I disliked Magmar's design, but Magmortar's is much worse.
Togekiss-I'd complain about it making sense, except Togetic's didn't either, so it's mostly fair game. Since the design isn't a bad one, being elegant but still strong, I don't mind this one.
Dusknoir-I don't dislike it, but don't see the point either. It learned to fly again, so it doesn't need legs, but how do you explain it getting a face on its stomach? This was purely an attempt to buff dusclops first and worked with the design later. Not bad, but feels unnatural.
Most of these evolutions aren't bad, but don't really feel like the natural progression of that evolutionary line. The only real exception is Kingdra, because it was created with Seadra in gen 1 but was scrapped until gen 2. My other opinions are more based on their design compared to the pre evolution, but I don't really think any (besides Kingdra) were necessary, and therefore somewhat violate the whole mentality of evolution.
Now to Megas:
Venusaur/Charizard Y/Mawile/Aggron/Abomosnow/Sceptile/Lupunny-Minor changes, but clearly stronger. Design-wise, I am okay with these ones. They're not going to be many people's favorites, but they're successful at their goal
Blastoise/Aerodactyl/Tyranitar/Garchomp/Lucario-Same as last group, but they took it too far. Blastoise's cannon is pretty ridiculous, and Aerodactyl/Tyranitar/Garchomp/Lucario became unreasonably spiky. This completely violates the simplistic nature most of Pokemon's best designs have.
Pinsir/Kangaskhan/Scizor/Heracross/Ampharos/Houndoom/Gardevoir/Mawile/Medicham/Banette/Absol/Slowbro/Sableye-Mostly the same save it some major differences. For the most part I like these, but there's a few (Specifically Medicham) that just didn't do it for me. This category is entirely based on personal preference, so I left it black because I know my opinions won't match anyone's exactly.
Charizard X/Alakazam/Gengar/Gyarados/Blaziken/Manectric/Swampert/Altaria/Salamence/Metagross/Audino-These ones actually feel like evolutions. They're pretty significant changes to the bases that make them unique, and depending on your preferences, can be either your favorite or least favorite of the bunch, just like regular pokemon. This catagory has the greatest potential for success or failure. For instance, I hate Swampert's design, and almost put it in the second group, but then I realized it was me judging it as its own unique design and deciding I didn't like it rather than me comparing it to its base and disapproving of the changes.