Heya,
I do single speedlite in the sun all the time. I use a 5.5" and a 10" reflector. I also use a 28" beauty dish. The dish needs an assistant or a stand, so not a run& gun option. I did however do flash + 5.5" reflector on a boom stick and I can totally run & gun with that at close distance with a wide angle lens and I prefer that. The 10" reflector is better in my opinion and I enjoy it, but, it's bigger, heavier, and even more directional, but it's still a run & gun option as you can set it up on a boom stick and hand hold it easily.
You lose a lot of output as your speedlite gets farther away from your subjects. And the speedlite cannot effectively fill larger modifiers without a lot of distance between them (such as a huge umbrella or softbox) with the speedlite head (fresnel?). They're great for fill light, even in full sun, at close proximity to subject. Reflectors are great as you can keep your output higher rather than lose a ton to a larger surface area. You just have to keep in mind the angular spread and if you get it too tight and too close, you get very isolating effects (which can be great, but not always ideal if you want a natural blended look).
That said, bare flash is fine if you're using it as fill. If it's proper fill you won't see hard spectral highlights really and it's a lot less to carry for run & gun. I do this often. I only throw a gel on there if we're in some warm light (1/4th CTO) (and despite the gimicky nature of magmod stuff, I really like the magmod gel system and I use it, it's so fast and simple , so I really enjoy that).
Here's my smallest self-carry run & gun, it's a just a flashpoint boom stick with a 5.5" reflector (bowens) with a speedlite on an s-bracket (bowens). Great for close range wide angle run & gun, which I do when we're out and especially in public areas and I want the weight down.
IMG_0947 by
Martin Wise, on Flickr
Here's how I use it in full sun at close range (ultrawide angle lets me get really close so I can get a lot of power retention from the speedlite) to match and/or simply fill, this is at F8, ISo 50, 1/200 (synch speed) so I don't lose power to HSS (just using TTL):
IMAGE LINK: https://flic.kr/p/WsR69X IMG_6064 by
Martin Wise, on Flickr
IMAGE LINK: https://flic.kr/p/WcSUgN IMG_6082 by
Martin Wise, on Flickr
Same concept, just 1/200s synch speed (to not lose power to HSS), base ISO, and let my aperture be variable. Full flash with the reflector at close range with an ultrawide angle:
IMAGE LINK: https://flic.kr/p/VSyjjN IMG_6123 by
Martin Wise, on Flickr
++++++++++++++
Here's bare flash, no modifier, set to ETTL and fill light (-2/3rds EC, +/- depending on what I want):
IMAGE LINK: https://flic.kr/p/WpcWH1 IMG_6168 by
Martin Wise, on Flickr
IMAGE LINK: https://flic.kr/p/VYQt68 IMG_6025 by
Martin Wise, on Flickr
IMAGE LINK: https://flic.kr/p/VYQtfX IMG_6017 by
Martin Wise, on Flickr
The farther you are from the subject, the harder the light is (see last example above, you can see spectral highlights, was too far away to soften it).
I always take a flash. I don't always take a modifier. Just adjust how you are lighting the subject based on what you have. For me, with speedlites, I'm trying to get close to subject and only go for fill (in sunlight). If I had a modifier, I can often match full sun and/or fill, and gain some distance.
I use bigger modifiers and strobes for purposeful sessions. For just going out with the girls, I just take a speedlite and either use a 5.5" or 10" reflector and sometimes my 28" beauty dish.
Very best,
. 
