No problems here what so ever with 55 devices in home. (basically all wall plugs and switches have shelly's).
Saying this I DID had troubles in the past from the Wifi but it wasnt from Shelly the problem. I have Unifi in home and sometime Unifi botches it in the AP firmwares. Lately they finally became stable with the latest EA versions.
Saying all the above i try to make you to resee your Wifi in home. Scan with your mobile the channels that your neighboors have and choose other channels to reduce the interference. I personally inside the house i have my Unifi's to work at 2.4ghz at channel number No8 which is crazy but this was the only channel with the best SNR and all my shelly's went under 70db signal. Also make an IoT SSID with only the necessary things checked (only 2.4ghz with Wpa2 and nothing else fancy stuff enabled).