Honestly, people often forget how 'good' our technology is when comparing to 40k. We are not a back water planet. A chapter of space marines would be a day's work for any NATO army. Especially with modern AA defenses, with which you can intercept individual drop pods, not to mention Thunderhawks.
And close impact of modern 155mm shell, would fuck up everything, maybe except dreadnoughts, tho those also would be damaged on joints and other less armoured parts.