No burden at all:
On the 5d2 I use the pentax 645 af 35mm and the following mamiya 645 manual lenses: 55mm, 80mm, 110mm and 150mm, also the schneider kreuznach 60mm curtagon and for shift plus tilt I use the apo-rodogon 105mm via the zörk tilt/shift adapter.
On the mamiya zd I use the zörk shift adapter with the following pentax 67 lenses: 45mm, 55mm and 75mm.
I am very happy with all of the lenses. also the 35mm pentax and the 45mm pentax 67 (while very sharp) have some unpleasant distortion when shifted.
For architecture photography the 35mm pentax and the 17mm canon shift are my workhorses, that stay on the 5d2 for 90% of the time. for interior and furniture photography the 55mm mamiya and on the mamiya ZD the 75mm pentax 67 are my favorite lenses. for closer-up produkt shots the 80mm mamiya and the 105mm apo-rodogon are hard to beat. they are also quite cheap to get. the zörk proshiftadapter is sometimes available on ebay, the shiftadapter for mamiya 645 is hard to get. i got mine specialy built from mr zörkendorfer. I think it cost about 1.500 euro or the like.
All in all, you pay a bit for the shiftadapters but than you can use a whole lot of excellent and very cheap lenses.
plus: When I don´t use the mamiya 645 lenses on the canon, I can use them strait on the mamiya ZD which is very handy if you want to travel light and still use two systems (35mm and medium format).