We've had a wedding in the family garden, and I've also been to 2 others; in all three instances, they were brilliant, and each had around 100 guests in a marquee. We have also done smaller events up to 30 guests for birthdays where we used a large gazebo (with sides) or the conservatory/downstairs areas and didn't bother with extra/outdoor toilets.
From experience of the one wedding we hosted, don't underestimate the work involved. We had the space and had a lawn area created specifically for the marquee, and it was a sibling that was getting married, organising the logistics, suppliers and making sure everything was ready took a considerable amount of time. This was done mainly by our parents, not the bride and groom, who mainly concentrated on themselves and enjoying the build-up and day.
The clear-up was also quite a lot, again done by my parents and the other siblings as the newlyweds had left for their honeymoon. It took the whole of Sunday.
Top tip: if you are doing the drinks/bar yourself, get a couple of experienced people to run it for you. We 'hired' a couple of our friends to do it (we were not getting married, so they wouldn't have been invited).
Cost: We had our reception at a local hotel, and whilst the food and drink bill was more, the overall costs of hosting the reception, once you had taken everything into account, were about the same.
The garden wedding was lovely and very much my siblings' vibe, but I didn't want the added pressure or to ask my parents to host again.