I have four boxes in my house - the oldest one is now having some problems, but as it only cost £25 three years ago, it's served it's time.
As above, the more you spend, the better the quality - I have no issues with any box-sets and films, and rarely suffer from any issues on the rare occasion that one of our games are shown on TV (the way we're playing at the moment, it's amazing anyone is watching them!). Managed to watch the entire Ryder Cup in full HD, with no buffering, but think football suffers a bit on quality with the streams sometimes, as there is so much information constantly changing, whereas golf is a fairly static game.
My Kodi is stripped right down and I only have five or six add-ons active. Sports Devil is the one I use for football, and can usually find the required stream with no issues - although some links do drop out, but it's no different than streaming from web-sites on your PC. Phoenix Sports also seems to be ok for Sky and BT games.
Windom is right, 4G is becoming a lot faster than wi-fi these days - I'm currently rolling out Vodafone and O2 4G networks rurally up here, and they are regularly getting speeds of 40-50Mbps, whilst I can get 80Mbps on my EE phone at home, whilst I only get 35Mbps through my wi-fi in the house!