I kinda like the way English affixes figure into the game (or games – affixes are as big, if not a bigger, part of OSKI tactics). More synthetic languages (like my Finnish) I think usually just won’t accept inflectional morphology as part of the game, as else the game would be broken, and that’s dull. English has enough 'fixes to make them matter and not enough to flood the game with them, and I think that’s a thing to be rejoiced about.
The way WYPS works makes it very much a game for tactical play. Not that you wouldn’t profit from wide vocabulary and hellishly long words, but if that’s all you care about, you’re bound to lose often against even moderately verbose opponents with a keen eye for clever play.
Serious high ranked players seldom leave straight-line words open for affixes. Even if the opponent does not have any
er or -ed or -ing or -s or un or a- (or whatever) ready in their hand, the open word is still a weakness that can come back haunting later in the game, when the opponent finally finds the right letters and you would rather play somewhere else.
The esses are really good letters. They are common all-around the words, they can be added as a suffix to almost any dictionary form noun or verb, they are pretty good for protecting your own words from suffixing ... and they form nice big consonant clusters, so their reach is unsurpassable. Still, one ess amounts for one trick and is rarely a game winner. Consonant/vowel balance often counts more, and I think I’d more often throw an ess away and keep an e than another way around.
More than any letter winning the game, there are letters that go a long way for losing games – the letter Q most of all.