Well, I've actually faced a pretty fierce Magcargo deck run by Bondi. Maybe that is why I think this is a great card mostly hurt by the resurgence of Gyarados decks and not much else.
Magcargo is a Fire Type, but that doesn't actually matter much. Being a Stage 1 is normally bad, but it has a useful Basic (Slugma) that can discard the top card from your deck, and if it is a :fire: Energy, you get to attach it to Slugma. Since swarming Basic Pokemon is relatively easy, this really speeds the decks set-up, even if you went second and use Broken Time Space to Evolve right after using the Poke-Power.
100 HP is a good, solid amount for a Stage 1. More would be nice, but a bit out of place. Unfortunately, I can already tell you what sabotages the deck I've seen utilize this card: the Water Weakness. I faced Magcargo when Gyarados and Rain Dance were still struggling. Granted, the person I faced had moved on already by the time those two decks experienced a tidal surge, so maybe a solution exists and I just don't know it. The lack of Resistance is annoying but as usual isn't a big deal, and what may surprise people is that the three Energy Retreat Cost isn't an issue either. This comes in part from the Energy acceleration, in part from the fact this is clearly a card meant to go down swinging, and in part due to a deck partner that makes you run a lot of changing out cards, anyway.
Searing Flame doesn't get used much. As in, almost never. Lava Flow is there. 60 a turn is lame, but you have just enough Energy acceleration that, even without additional combos, you should be able to attach and discard an Energy for a turn or two to do 80 each of those hits, then right before you'd be KO'd blow it all for a big hit.
That isn't enough for the modern Modified format, is it? Here's where the rest of the deck comes in. First things first, a deck that needs to run a lot of Fire Energy begs for Ninetales with Roast Reveal. Two of them on the Bench and you'll quickly set-up everything else and also important, help you filter out most non-Fire Energy cards. The final Pokemon piece of the puzzle is Heatran Lv.X so that you can freely discard two Fire Energy each turn and still build energy. 100 damage a turn while still building up another Pokemon on the bench is pretty handy. Modern versions of the deck could toy with Expert Belt/Seeker to try some Prize denial after firing off Magcargo's Fire Energy, but even 120 HP doesn't last long in this format. Oh, and if you're wondering, Heatran Lv.X necessitates the deck run a lot of cards to Bench it, hence why I wasn't worried about retreating Magcargo.
Ugh, it's been a long day for me. This review is pretty poor, but I needed to state the good this card has going for it. Hopefully I can clean it up before it gets locked.
Ratings
Modified: 7/10 - Only this low because of a problem Weakness.
Limited: 8/10 - Only drawback here is you have to dedicate your deck to Fire Energy, and you need to be careful what Slugma discards from your deck.