i watched 2 matches only last night:

spain-czech... 100%,

sweden-ireland... buffered twice, stop, exit, go back in, ok.

i also tried cctv5 - it's stable but video quality not as good as tsn1.

i'm using adrian supertv tsn1 on smart tv, 30mbps, wifi.

same result if i use android box, ethernet.
from the various comments, users are experiencing diff when using different addons, addon versions and links for tsn.

diff may also come from internet speed and device type.

check...

.. supertv... do not use the euro2016 links; use world of sports-sports24/7-tsn1.
.. check supertv version, latest is 1.0.7.
.. clear kodi cache.
.. do speedtest, see if incoming stable/normal; can power off modem, router, restart.
no experience with htpc but on pc, i find it less smooth, suspect the firewalls obstructing, and rightly so.