First,
It's a matter of preserving Chomp throughout the match. As an example to how you would do this, I'll use Mega Scizor and Goodra. In my hypothetical situation we've created, I have a team that Scizor, with different moves, can take apart. Goodra is the only pokemon on my team that can take a +2 Bullet Punch and defeat Scizor with flamethrower. As a player, I have to realize that if I bring in Goodra and let it get damaged, I will lose. If it is within my control, which excludes phazing, I will not bring in Goodra until Scizor has to be dealt with, which I will have to do with a safe switch, usually after sacking something, because Goodra can and will die if I switch it on an attacking move.
It's the same type of situation here. You have to preserve your check, or you'll lose. This means there is no way in hell I'm switching in my Garchomp on MegaWile if the possibility stands I will be attacked on the switch by said Mawile and I'm out one check. Alternatively, you run multiple checks if they have synergy and it wouldn't hinder your team to do so.
On the sacking issue, I believe Keldeo is in the same situation, as if it gets hit by Fire Fang by +2 MegaWile it's in KO range for Sucker Punch. What it boils down to is Keldeo shouldn't be switching in on Mawile unless you're feeling risky or you can guarantee you won't be Fire Fanged, or Play Roughed. Garchomp is in a more extreme version of this situation where it shouldn't switch in until it's ready to do its job. So, even if you are spamming Play Rough, that's great, but when Chomp gets a free switch because something died to Mawile, just look out.
On the matter of entry hazards, I'm not saying this is a perfect solution, but there are spinners and foggers that are faster than Mawile and can clear away the hazards so Garchomp has its precious health retained. Yes, the fogger/spinner will very likely die in the process, and I'm pretty sure we've established that very few Pokemon can take a hit from this physical nuke at this point in the discussion.
Also, the handling SD rather well was a comparative statement, as it appears to be doing a better job than a large portion of the metagame. As I stated, very few pokemon can boast of being able to come in versus a Mawile and in a favorable situation, net the KO. If I made it sound like it would be coming in without a safe switch, that's on me, and I should have worded my statement better.
tl;dr It's not as bad as you make it out to be, though I have admitted and continually will that it is flawed and shouldn't be your first choice. However, the choice to use MegaChomp as a check, however shaky, exists.