How do we feel about picnic tables at the reception?

This would be hard and frustrating for guests of varying ages and since most people dress nicely for a wedding I would feel very awkward trying to get in and out of a table in a dress. Nope no thanks

Hard pass. I have a bad knee and have trouble getting in and out of picnic tables, even when I'm not wearing a dress...

I think this would work for only the most casual of casual weddings.

Like I am picturing you in a sundress and your FH in shorts.

See if you can rent a few folding chairs for anyone that possibly can't maneuver a picnic table.

Short Answer: Yes - it would be a deal breaker.

What is your back up plan if it's raining or inclement weather?

As long as it fits your vision and how you want your wedding to be it sounds perfect!

A rainy wedding would still be really unpleasant even under a shelter, IMO.

Yeah deal breaker for me, too. There are a lot of things that look nice on pinterest that don't work in the real world and this is one of them. At best you have people awkwardly getting in and out in their dresses (and possibly dirtying them) and at worst you aggravate an injury or someone has to leave because of how uncomfortable they are.

This exactly.

Roofs don't always protect you from getting wet - especially guests who are seated on the sides closest to the outside. I take public transit and even when I stand under the awning at a bus stop I can still get damp because the wind may blow the rain around or the water dripping off the roof can splash down on you. There are more factors to consider than just a roof that keeps you mostly dry - but not all the way dry